document gdb.init check only performed in home directory

document gdb.init check only performed in home directory

The documentation is a little ambiguous and might be suggesting that
GDB checks all locations where a gdbinit might be located for gdb.ini
files.  This clarifies the current situation.

gdb/doc/ChangeLog:

        * gdb.texinfo (Startup): Minor rewording, and clarify scope of
        gdb.ini warning for Windows port.