ld/doc: Document that multiple MEMORY commands are allowed.

ld/doc: Document that multiple MEMORY commands are allowed.

The linker documentation explicitly states that there can be only one
MEMORY command.  This is not true.  Multiple MEMORY commands are
allowed, the contents of all will be treated as if a single MEMORY
command was given.

Update the documentation to make this clear to the users.

ld/ChangeLog:

	* ld.texinfo (MEMORY): Explain that multiple MEMORY commands are
	acceptable.