input/csv: update comments/helptext for analog input data

Expand the developer comment that's inline in the source file. There is
enough room to explain things. Try to come up with a "one-line" help text
that is precise yet compact, and does inform the user of available format
choices including modifiers. Chances are that longer descriptions start
reducing the usefulness of the help or the visibility of options when
users are in a hurry. Those who care can access the manual.

Mark more options as obsolete, and mention more default values in the
builtin help text. Also tweak a comment on getting channel names from
header lines.

src/input/csv.c

@@ -41,9 +41,10 @@
  *     "all remaining columns", only applicable to the last field), a format
  *     specifying character ('x' hexadecimal, 'o' octal, 'b' binary, 'l'
  *     single-bit logic), and an optional bit count (translating to: logic
- *     channels communicated in that column). This "column_formats" option
+ *     channels communicated in that column). The 'a' format marks analog
- *     is most versatile, other forms of specifying the column layout only
+ *     data, an optionally following number is the digits count (resolution).
- *     exist for backwards compatibility.
+ *     This "column_formats" option is most versatile, other forms of
+ *     specifying the column layout only exist for backwards compatibility.
  *
  * single_column: Specifies the column number which contains the logic data
  *     for single-column mode. All logic data is taken from several bits
@@ -109,6 +110,9 @@
  * - ... -I csv:start_line=20:header=yes:...
  *   Skip the first 19 text lines. Use line 20 to derive channel names.
  *   Data starts at line 21.
+ * - ... -I csv:column_formats=*a6 ...
+ *   Each column contains an analog value with six significant digits
+ *   after the decimal period.
  */
 
 /*
@@ -605,7 +609,7 @@ static int make_column_details_from_format(const struct sr_input *in,
 			if (!detail->text_format)
 				continue;
 			/*
-			 * Create channels with appropriate names. Optionally
+			 * Pick most appropriate channel names. Optionally
 			 * use text from a header line (when requested by the
 			 * user). In the absence of header text, channels are
 			 * assigned rather generic names.
@@ -1451,17 +1455,17 @@ enum option_index {
 static struct sr_option options[] = {
 	[OPT_COL_FMTS] = {
 		"column_formats", "Column format specs",
-		"Specifies text columns data types: comma separated list of [<cols>]<fmt>[<bits>], with -/x/o/b/l format specifiers.",
+		"Specifies text columns data types: A comma separated list of [<cols>]<fmt>[<bits>] items, with - to ignore columns, x/o/b/l for logic data, a (and resolution) for analog data.",
 		NULL, NULL,
 	},
 	[OPT_SINGLE_COL] = {
 		"single_column", "Single column",
-		"Enable single-column mode, exclusively use text from the specified column (number starting at 1).",
+		"Enable single-column mode, exclusively use text from the specified column (number starting at 1). Obsoleted by 'column_formats'.",
 		NULL, NULL,
 	},
 	[OPT_FIRST_COL] = {
 		"first_column", "First column",
-		"Number of the first column with logic data in simple multi-column mode (number starting at 1, default 1).",
+		"Number of the first column with logic data in simple multi-column mode (number starting at 1, default 1). Obsoleted by 'column_formats'.",
 		NULL, NULL,
 	},
 	[OPT_NUM_LOGIC] = {
@@ -1471,7 +1475,7 @@ static struct sr_option options[] = {
 	},
 	[OPT_FORMAT] = {
 		"single_format", "Data format for simple single-column mode.",
-		"The number format of single-column mode input data: bin, hex, oct.",
+		"The number format of single-column mode input data: bin, hex, oct. Obsoleted by 'column_formats'.",
 		NULL, NULL,
 	},
 	[OPT_START] = {
@@ -1481,12 +1485,12 @@ static struct sr_option options[] = {
 	},
 	[OPT_HEADER] = {
 		"header", "Get channel names from first line.",
-		"Use the first processed line's column captions (when available) as channel names.",
+		"Use the first processed line's column captions (when available) as channel names. Off by default",
 		NULL, NULL,
 	},
 	[OPT_RATE] = {
 		"samplerate", "Samplerate (Hz)",
-		"The input data's sample rate in Hz.",
+		"The input data's sample rate in Hz. No default value.",
 		NULL, NULL,
 	},
 	[OPT_DELIM] = {
@@ -1496,7 +1500,7 @@ static struct sr_option options[] = {
 	},
 	[OPT_COMMENT] = {
 		"comment_leader", "Comment leader character",
-		"The text which starts comments at the end of text lines.",
+		"The text which starts comments at the end of text lines, semicolon by default.",
 		NULL, NULL,
 	},
 	[OPT_MAX] = ALL_ZERO,