doc: fix ambiguities in logname(1) and whoami(1)

* doc/coreutils.texi (whoami invocation): Clarify it prints names,
not numeric IDs.
* man/whoami.x: Likewise.
* man/logname.x: Reference getlogin(3).
* src/logname.c: Clarify that it prints the login name,
rather than the name of the effective user ID.

Fixes https://bugs.gnu.org/48894

doc/coreutils.texi

@@ -15643,10 +15643,10 @@ options}.
 
 
 @node whoami invocation
-@section @command{whoami}: Print effective user ID
+@section @command{whoami}: Print effective user name
 
 @pindex whoami
-@cindex effective user ID, printing
+@cindex effective user name, printing
 @cindex printing the effective user ID
 
 @command{whoami} prints the user name associated with the current

man/logname.x

@@ -2,3 +2,5 @@
 logname \- print user\'s login name
 [DESCRIPTION]
 .\" Add any additional description here
+[SEE ALSO]
+getlogin(3)

man/whoami.x

@@ -1,4 +1,4 @@
 [NAME]
-whoami \- print effective userid
+whoami \- print effective user name
 [DESCRIPTION]
 .\" Add any additional description here

src/logname.c

@@ -38,7 +38,7 @@ usage (int status)
     {
       printf (_("Usage: %s [OPTION]\n"), program_name);
       fputs (_("\
-Print the name of the current user.\n\
+Print the user's login name.\n\
 \n\
 "), stdout);
       fputs (HELP_OPTION_DESCRIPTION, stdout);