cherry picking documentation patches to 8.1 branch

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

hello, i'd like to backport these patches into the 8.1 branch:

https://sourceware.org/ml/gdb-patches/2018-01/msg00259.html
https://sourceware.org/ml/gdb-patches/2018-01/msg00266.html

commit dbfa452355153093d4d6841515f9891c9572113f
Author:     Szabolcs Nagy <szabolcs.nagy@arm.com>
AuthorDate: 2018-01-15 10:44:12 +0000
Commit:     Szabolcs Nagy <szabolcs.nagy@arm.com>
CommitDate: 2018-01-15 11:43:33 +0000

    Fix gdb.texinfo for old makeinfo (again)

    With old makeinfo (version 4.13) the changes introduced in

    commit ba643918cf869fa0d064d733f69b453b6fe642ea
    Author:     Sergio Durigan Junior <sergiodj@redhat.com>

        Install and generate docs for gdb-add-index

    fail to build with

    gdb/doc/gdb.texinfo:2498: warning: `.' or `,' must follow @xref, not `@'.
    gdb/doc/gdb.texinfo:2517: warning: `.' or `,' must follow @xref, not `@'.
    gdb/doc/gdb.texinfo:43443: Node `gdb-add-index man' requires a sectioning command (e.g., @unnumberedsubsec).
    gdb/doc/gdb.texinfo:43443: `gdb-add-index man' has no Up field (perhaps incorrect sectioning?).
    gdb/doc/gdb.texinfo:43350: Next field of node `gdbinit man' not pointed to (perhaps incorrect sectioning?).
    gdb/doc/gdb.texinfo:43443: This node (gdb-add-index man) has the bad Prev.

    This patch fixes the warnings too.

    gdb/doc/ChangeLog:

            * gdb.texinfo (set cwd): Add period.
            (gdb-add-index man): Move anchor.


and

commit d726cb5d3784f7ed96318a2214ea777a86f9faad
Author:     Szabolcs Nagy <szabolcs.nagy@arm.com>
AuthorDate: 2018-01-15 13:06:45 +0000
Commit:     Szabolcs Nagy <szabolcs.nagy@arm.com>
CommitDate: 2018-01-15 13:28:25 +0000

    Fix gdb.texinfo more

    Add incorrectly removed @pindex back.

    gdb/doc/ChangeLog:

            * gdb.texinfo (gdb-add-index man): Add pindex.