doc: fix SVR3/4 references.

doc: fix SVR3/4 references.

Consider:
  $ git grep SVR4 861325a84f231a2a2da307b53ef5ff527d67e876 | cut -c 42-
  ChangeLog:	that Jim developped stackovf.c on a 486 running SVR4.0 (ESIX), and
  ChangeLog:	`boolean'.  This tag conflicts with <sys/types.h> on SVR4.
  README:processor.  It is mostly SVR4 compatible, although it has some
  doc/m4.texinfo:processor.  It is mostly SVR4 compatible, although it has some
  examples/WWW/m4lib/whatis.m4:version, and SVR4, although it has some extensions (for example,
  src/stackovf.c:   signal stacks using either SVR4 or BSD interfaces.
  src/stackovf.c:      1. The siginfo parameter (with siginfo.h, i.e., SVR4).
  src/stackovf.c:/* SVR4.  */
In particular:
  doc/m4.texinfo:processor.  It is mostly SVR4 compatible, although it has some
Elsewhere in `doc/m4.texinfo', only `Release 3' is referenced:
  $ git grep Release 861325a84f231a2a2da307b53ef5ff527d67e876 -- doc/m4.texinfo | cut -c 42-
  doc/m4.texinfo:System V, Release 3 version, except for some minor differences.
  doc/m4.texinfo:UNIX, such as System V Release 3, Solaris, and @acronym{BSD} flavors.
This commit simply changes those last 2 lines by replacing `Release 3'
with `Release 4'.
* doc/m4.texinfo: Note that m4 is mostly compatible with SVR4.

Signed-off-by: Michael Witten <mfwitten@gmail.com>