Improve documentation and UI of 'C-x v L'

* lisp/vc/vc.el (vc-print-root-log): Improve the wording of
the doc string and of the prompt for the root directory.

* etc/NEWS: Improve and expand the wording of the changes in
'C-x v L'.

* doc/emacs/maintaining.texi (VC Change Log): Improve and
clarify wording of the 'C-x v L' description.

3 files changed, 19 insertions, 7 deletions

diff --git a/doc/emacs/maintaining.texi b/doc/emacs/maintaining.texi
index c1f7aed114f..66fa54113f9 100644
--- a/doc/emacs/maintaining.texi
+++ b/doc/emacs/maintaining.texi
@@ -993,8 +993,14 @@ version-controlled directory tree (RCS, SCCS, CVS, and SRC do not
 support this feature).  With a prefix argument, the command prompts
 for the maximum number of revisions to display.  A numeric prefix
 argument specifies the maximum number of revisions without prompting.
-When the numeric prefix argument is @kbd{M-1}, the command prompts
-for the revision ID, and displays its log entry with a diff of changes.
+When the numeric prefix argument is 1, as in @w{@kbd{C-1 C-x v L}} or
+@w{@kbd{C-u 1 C-x v L}}, the command prompts for the revision ID, and
+displays the log entry of that revision together with the changes
+(diffs) it introduced.  (Some less capable version control systems,
+such as RCS and CVS, don't have commands to show a revision log with
+its diffs; for them the command displays only the log entry, and you
+can request to show the diffs by typing @kbd{d} or @kbd{D}, see
+below.)
 
   The @kbd{C-x v L} history is shown in a compact form, usually
 showing only the first line of each log entry.  However, you can type
diff --git a/etc/NEWS b/etc/NEWS
index 688b3f85cbb..57dbc9324a1 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -998,7 +998,12 @@ With a prefix argument asks for a command, so for example,
 just one log entry found by its revision number.
 
 +++
-*** 'C-1 C-x v L' asks for a revision and shows its log entry with diff.
+*** It is now possible to display a specific revision given by its ID.
+If you invoke 'C-x v L' ('vc-print-root-log') with a numeric argument
+of 1, as in 'C-1 C-x v L' or 'C-u 1 C-x v L', it asks for a revision
+ID, and shows its log entry together with the diffs introduced by the
+revision's commit.  (For some less capable VCSes, only the log entry
+is shown.)
 
 *** 'C-x v =' can now mimic Magit's diff format.
 Set the new user option 'diff-font-lock-prettify' to t for that, see
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index d0d2c39ac3d..5ac07e78891 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -2481,13 +2481,14 @@ WORKING-REVISION and LIMIT."
 
 ;;;###autoload
 (defun vc-print-root-log (&optional limit revision)
-  "List the change log for the current VC controlled tree in a window.
+  "List the revision history for the current VC controlled tree in a window.
 If LIMIT is non-nil, it should be a number specifying the maximum
 number of revisions to show; the default is `vc-log-show-limit'.
 When called interactively with a prefix argument, prompt for LIMIT.
 When the prefix argument is a number, use it as LIMIT.
-A special case is when the prefix argument is 1, in this case
-it asks for the revision and shows it with its diff."
+A special case is when the prefix argument is 1: in this case
+the command asks for the ID of a revision, and shows that revision
+with its diffs (if the underlying VCS supports that)."
   (interactive
    (cond
     ((eq current-prefix-arg 1)
@@ -2517,7 +2518,7 @@ it asks for the revision and shows it with its diff."
          rootdir)
     (if backend
         (setq rootdir (vc-call-backend backend 'root default-directory))
-      (setq rootdir (read-directory-name "Directory for VC root-log: "))
+      (setq rootdir (read-directory-name "Directory for VC revision log: "))
       (setq backend (vc-responsible-backend rootdir))
       (unless backend
         (error "Directory is not version controlled")))