Improve MI -dprintf-insert documentation

I found the documentation for -dprintf-insert a bit unclear.  It
didn't mention the possibility of multiple arguments, and I also
noticed that it implied that the format parameter is optional, which
it is not.

While looking into this I also noticed a few comments in the
implementation that could also be improved.

Then, I noticed a repeated call to strlen in a loop condition, so I
fixed this up as well.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>

gdb/doc/gdb.texinfo

@@ -32328,10 +32328,15 @@ times="0"@}]@}
 @smallexample
  -dprintf-insert [ -t ] [ -f ] [ -d ] [ --qualified ]
     [ -c @var{condition} ] [--force-condition] [ -i @var{ignore-count} ]
-    [ -p @var{thread-id} ] [ @var{locspec} ] [ @var{format} ]
-    [ @var{argument} ]
+    [ -p @var{thread-id} ] [ @var{locspec} ] @var{format}
+    [ @var{argument}@dots{} ]
 @end smallexample
 
+@noindent
+Insert a new dynamic print breakpoint at the given location.
+@xref{Dynamic Printf}.  @var{format} is the format to use, and any
+remaining arguments are passed as expressions to substitute.
+
 @noindent
 If supplied, @var{locspec} and @code{--qualified} may be specified
 the same way as for the @code{-break-insert} command.

gdb/mi/mi-cmd-break.c

@@ -93,8 +93,8 @@ setup_breakpoint_reporting (void)
 }
 
 
-/* Convert arguments in ARGV to the string in "format",argv,argv...
-   and return it.  */
+/* Convert arguments in ARGV to a string suitable for parsing by
+   dprintf like "FORMAT",ARG,ARG... and return it.  */
 
 static std::string
 mi_argv_to_format (const char *const *argv, int argc)
@@ -102,9 +102,9 @@ mi_argv_to_format (const char *const *argv, int argc)
   int i;
   std::string result;
 
-  /* Convert ARGV[OIND + 1] to format string and save to FORMAT.  */
+  /* Convert ARGV[0] to format string and save to FORMAT.  */
   result += '\"';
-  for (i = 0; i < strlen (argv[0]); i++)
+  for (i = 0; argv[0][i] != '\0'; i++)
     {
       switch (argv[0][i])
 	{
@@ -151,7 +151,7 @@ mi_argv_to_format (const char *const *argv, int argc)
     }
   result += '\"';
 
-  /* Apply other argv to FORMAT.  */
+  /* Append other arguments.  */
   for (i = 1; i < argc; i++)
     {
       result += ',';