gdb/doc: Explain linker namespaces

Recent GDB commits added more features related to linker namespaces and
documented them on the manual, but did not add a convenient way for a
user to understand what they are. This commit adds a quick explanation
of what they are.

It also fixes the inconsistency of using "linker namespaces" and
"linkage namespaces", by always using the first form to avoid user
confusion.

Approved-By: Eli Zaretskii <eliz@gnu.org>