doc: ls: clarify description of timestamps

* src/ls.c (usage): Don't mention "modification" in the
description of ctime (-c), as it's confusing with mtime.
Mention "metadata" when discussing "change" time to
disambiguate from data change time.
* doc/coreutils.texi (ls invocation): State that --time=creation
falls back to using mtime where not available.

doc/coreutils.texi

@@ -8031,7 +8031,8 @@ sort according to the atime.
 @opindex --time
 @opindex birth time@r{, printing or sorting files by}
 @opindex creation timestamp@r{, printing or sorting files by}
-In long format, print the file creation timestamp if available.
+In long format, print the file creation timestamp if available,
+falling back to the file modification timestamp (mtime) if not.
 When sorting by time or when not using long format,
 sort according to the birth time.
 @xref{File timestamps}.

src/ls.c

@@ -5423,7 +5423,7 @@ Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.\n\
 "), stdout);
       fputs (_("\
   -c                         with -lt: sort by, and show, ctime (time of last\n\
-                             modification of file status information);\n\
+                             change of file status information);\n\
                              with -l: show ctime and sort by name;\n\
                              otherwise: sort by ctime, newest first\n\
 \n\
@@ -5557,7 +5557,7 @@ Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.\n\
       fputs (_("\
       --time=WORD            change the default of using modification times;\n\
                                access time (-u): atime, access, use;\n\
-                               change time (-c): ctime, status;\n\
+                               metadata change time (-c): ctime, status;\n\
                                birth time: birth, creation;\n\
                              with -l, WORD determines which time to show;\n\
                              with --sort=time, sort by WORD (newest first)\n\