docs: improve the description of which rc-files are read during startup

doc/nano.1

@@ -331,6 +331,8 @@ hard-wrapping of long lines, \fBM\-S\fR toggles soft-wrapping,
 See at the end of the \fB^G\fR help text for a complete list.
 
 .SH FILES
+When \fB\-\-rcfile\fR is given, \fBnano\fR will read just the specified file
+for setting its options and syntaxes and key bindings.  Without that option,
 \fBnano\fR will read two configuration files: first the system's
 \fInanorc\fR (if it exists), and then the user's \fInanorc\fR (if it
 exists), either \fI~/.nanorc\fR or \fI$XDG_CONFIG_HOME/nano/nanorc\fR

doc/nano.texi

@@ -690,10 +690,13 @@ The following global toggles are available:
 
 The nanorc files contain the default settings for @command{nano}.  They
 should be in Unix format, not in DOS or Mac format.  During startup,
-@command{nano} will first read the system-wide settings, from /etc/nanorc
-(the exact path might be different), and then the user-specific settings,
+if @option{--rcfile} is not given, @command{nano} will read two files:
+first the system-wide settings, from @file{/etc/nanorc} (the exact path
+might be different on your system), and then the user-specific settings,
 either from @file{~/.nanorc} or from @file{$XDG_CONFIG_HOME/nano/nanorc}
 or from @file{.config/nano/nanorc}, whichever exists first.
+If @option{--rcfile} is given, @command{nano} will read just the
+specified settings file.
 
 A nanorc file accepts a series of "set" and "unset" commands, which can
 be used to configure @command{nano} on startup without using command-line

doc/nanorc.5

@@ -30,13 +30,16 @@ If you want the old, Pico behavior back, you can use \fBset breaklonglines\fR,
 \fBset jumpyscrolling\fR, and \fBset emptyline\fR.
 
 .SH DESCRIPTION
-The \fInanorc\fP file contains the default settings for \fBnano\fP, a
-small and friendly editor.  The file should be in Unix format, not in
-DOS or Mac format.  During startup, \fBnano\fP will first read the
+The \fInanorc\fP files contain the default settings for \fBnano\fP, a
+small and friendly editor.  They should be in Unix format, not in
+DOS or Mac format.  During startup, if \fB\-\-rcfile\fR is not given,
+\fBnano\fR will read two files: first the
 system-wide settings, from \fI/etc/nanorc\fP (the exact path might be
 different on your system), and then the user-specific settings, either
 from \fI~/.nanorc\fR or from \fI$XDG_CONFIG_HOME/nano/nanorc\fR
 or from \fI~/.config/nano/nanorc\fR, whichever is encountered first.
+If \fB\-\-rcfile\fR is given, \fBnano\fR will read just the specified
+settings file.
 
 .SH OPTIONS
 The configuration file accepts a series of \fBset\fP and \fBunset\fP