Fix makeinfo warnings in gdb.texinfo and python.texi docs

Building gdb-9.1 on a system that has an older version of makeinfo
(4.8) shows the following warnings:

-----------------
make[4]: Entering directory '/home/tester/gdb-9.1/build/gdb/doc'
makeinfo --split-size=5000000 --split-size=5000000   -I
../../../gdb/doc/../../readline/readline/doc -I ../../../gdb/doc/../mi
-I ../../../gdb/doc \
    -o gdb.info ../../../gdb/doc/gdb.texinfo
../../../gdb/doc/gdb.texinfo:21867: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21867: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21868: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21868: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21869: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21869: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21872: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21872: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21874: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21874: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21876: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21876: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21879: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21879: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21931: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21931: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21933: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21933: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21936: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21936: warning: unlikely character ] in @var.
../../../gdb/doc/gdb.texinfo:21939: warning: unlikely character [ in @var.
../../../gdb/doc/gdb.texinfo:21939: warning: unlikely character ] in @var.
../../../gdb/doc//python.texi:3297: warning: `.' or `,' must follow
@xref, not `A'.
make[4]: Leaving directory '/home/tester/gdb-9.1/build/gdb/doc'
-----------------

These are thrown by expressions like `@var{[host]}`, intended to
produce `[HOST]`.

In that context this should instead be changed to `[@var{host}]`, which
has the same effect but without the warnings.

As for the warning in `python.texi`, there's period missing at the end
of one `@xref{}` clause.  Added.

gdb/doc/ChangeLog:

2020-04-15  Artur Shepilko  <nomadbyte@gmail.com>

	* gdb.texinfo: Transform @var{[host]} to [@var{host}]; this
	clears makeinfo warnings.
	* python.texi: Add a missing period trailing an @xref{} clause;
	this clears a makeinfo warning.