[gdb/doc] Fix address location with file prefix

[gdb/doc] Fix address location with file prefix

In the docs I read [1]:
...
Address locations indicate a specific program address.  They have the
generalized form *address.

funcaddr

    An address of a function or procedure derived from its name.
    ...

'filename':funcaddr

    Like funcaddr above, but also specifies the name of the source file
    explicitly.  This is useful if the name of the function does not specify
    the function unambiguously, e.g., if there are several functions with
    identical names in different source files.
...

This is incorrect, the notation is in fact 'filename'::funcaddr.

Fix this by correcting the typo, and add a reference to "variable name
conflict", where the concept is explained in more detail.

Approved-By: Eli Zaretskii <eliz@gnu.org>
PR gdb/32748
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32748

[1] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Address-Locations.html