ld: More documentation for --defsym

ld: More documentation for --defsym

The ordering of command line options --defsym and -T is important,
however, the description of --defsym in the manual doesn't mention
this.

This commit adds more text to the description of --defsym to try and
explain this ordering requirement.

ld/ChangeLog:

	* ld.texi (Options): Extend the description of --defsym.