Add documentation for -Wflex-array-member-not-at-end.

'-Wflex-array-member-not-at-end (C and C++ only)'
     Warn when a structure containing a C99 flexible array member as the
     last field is not at the end of another structure.  This warning
     warns e.g.  about

          struct flex  { int length; char data[]; };
          struct mid_flex { int m; struct flex flex_data; int n; };

gcc/ChangeLog:

	* doc/invoke.texi (-Wflex-array-member-not-at-end): Document
	new option.