Use \*(GR in a few places in the body text.

One of them was not italicized.  The others produce the same output.
This change doesn't touch "groff" in section headers where it's set in
bold italic or where all the header is already set in italic.

doc/troff/doc.tr

@@ -6993,7 +6993,7 @@ to the \fB&C\fR and \fB&g\fR registers.
 Nevertheless it is not recommend to do so
 without reason.
 For instance macro packages read \fB\en(.g\fR to test
-if they are processed with groff.
+if they are processed with \*[GR.]
 Setting \fB&g\fR to \fB1\fR with the \fB&nr\fR request
 also has some side effects in Heirloom \*(TR:
 .de ListBegin
@@ -7013,7 +7013,7 @@ also has some side effects in Heirloom \*(TR:
 .ListBegin
 .tr &&
 .ListItem
-\fIgroff\fR\^'s notation for accessing symbols with
+\*(GR\^'s notation for accessing symbols with
 \fB\e[char\fIn\fB]\fR and \fB\e[u\fIXXXX\fB]\fR
 are enabled.
 .ListItem
@@ -7033,7 +7033,7 @@ leads to a different placement of the middle title element
 with \*(NR under certain conditions.
 This is emulated by setting \fB.g\fR to 1.
 .ListItem
-\fIgroff\fR\^'s left italic correction escape \fB\e,\fR is removed
+\*(GR\^'s left italic correction escape \fB\e,\fR is removed
 from the input instead of producing a \(oq\fB,\fR\(cq.
 .ListEnd
 .tr &.