gdb/doc: add '@:' after 'e.g.' to help texinfo

Add '@:' after 'e.g.' to let texinfo know that this is not the end of
a sentence.  This is only needed when there's a space immediately
after the last '.'.

gdb/doc/ChangeLog:

	* gdb.texi (Initialization Files): Add '@:' after 'e.g.'.
	(Source Path): Likewise.
	(GDB/MI Development and Front Ends): Likewise.
	(ARM Features): Likewise.
	(gdb man): Likewise.