When an option is shared between multiple front ends, the output of gcc -v --help uses the documentation string from the first front end for which it is listed.

Ada is listed before C, and thus all of the shared Ada/C options use the Ada documentation, rather than the C documentation.  Unfortunately, the Ada lang.opt file doesn't have good documentation for some of these:

---------------------------------------
The Ada front end recognizes the following options:

  -I                          This switch lacks documentation
  -Wall                       This switch lacks documentation
  -Wlong-long                 Do not warn about using "long long" when -pedantic
  -Wmissing-format-attribute  Warn about functions which might be candidates
                              for format attributes
  -Wmissing-prototypes        Warn about global functions without prototypes
  -Wold-style-definition      Warn if an old-style parameter definition is used
  -Wstrict-prototypes         Warn about unprototyped function declarations
  -Wvariadic-macros           Do not warn about using variadic macros when
                              -pedantic
  -Wwrite-strings             In C++, nonzero means warn about deprecated
                              conversion from string literals to `char *'.  In
                              C, similar warning, except that the conversion is
                              of course not deprecated by the ISO C standard.
  -fRTS=                      This switch lacks documentation
  -gnat<options>              Specify options to GNAT
  -gnatO                      This switch lacks documentation
  -nostdinc                   Do not search standard system include directories
                              (those specified with -isystem will still be used)
  -nostdlib                   This switch lacks documentation
---------------------------------------

My personal opinion is that this would really be best fixed by listing C and C++ first in the --help output, since they're much more commonly used.  However, a workaround would be copying the C documentation to the Ada lang.opt file.