tweaks: specifically refer to the manual of GNU grep for more regex info

doc/nano.texi

@@ -1000,7 +1000,8 @@ The period @code{.} matches any single character,
 @code{\<} matches the start of a word, and @code{\>} the end,
 and @code{\s} matches a blank.
 It also means that lookahead and lookbehind are not possible.
-A complete explanation can be found in @code{info grep regular}.
+A complete explanation can be found in the manual of GNU grep:
+@code{info grep regular}.
 
 A separate syntax can be defined for each kind of file
 via the following commands in a nanorc file:

doc/nanorc.5

@@ -346,7 +346,8 @@ The period \fB.\fR matches any single character,
 \fB\\<\fR matches the start of a word, and \fB\\>\fR the end,
 and \fB\\s\fR matches a blank.
 It also means that lookahead and lookbehind are not possible.
-A complete explanation can be found in \fBman grep\fR.
+A complete explanation can be found in the manual page of GNU grep:
+\fBman grep\fR.
 .sp
 For each kind of file a separate syntax can be defined
 via the following commands: