[Rd] ?gettext on NULL as domain argument

[Suharto Anggono Suharto Anggono]

In gettext.Rd in current (r81999) R devel (https://svn.r-project.org/R/trunk/src/library/base/man/gettext.Rd) or R 4.2.0 alpha (https://svn.r-project.org/R/branches/R-4-2-branch/src/library/base/man/gettext.Rd), description of 'domain' argument is
the 'domain' for the translation, a character string, or NULL to flush the cache; see 'Details'.

Describing domain = NULL as "to flush the cache" is partly wrong because it applies only to function 'bindtextdomain'. It doesn't apply to functions 'gettext' and 'ngettext' that are also documented by gettext.Rd and also have 'domain' argument. In 'gettext' or 'ngettext', domain = NULL is the default and means that domain is inferred.