update: note migrations may be performed.

As requested in Homebrew/homebrew-core#9316.

Author: MikeMcQuaid

Library/Homebrew/cmd/update.sh

@@ -1,6 +1,6 @@
 #:  * `update` [`--merge`] [`--force`]:
 #:    Fetch the newest version of Homebrew and all formulae from GitHub using
-#:    `git`(1).
+#:    `git`(1) and perform any necessary migrations.
 #:
 #:    If `--merge` is specified then `git merge` is used to include updates
 #:    (rather than `git rebase`).

docs/brew.1.html

@@ -398,7 +398,7 @@ <h2 id="COMMANDS">COMMANDS</h2>
 <code>pin</code>.</p></dd>
 <dt><code>untap</code> <var>tap</var></dt><dd><p>Remove a tapped repository.</p></dd>
 <dt><code>update</code> [<code>--merge</code>] [<code>--force</code>]</dt><dd><p>Fetch the newest version of Homebrew and all formulae from GitHub using
-<code>git</code>(1).</p>
+<code>git</code>(1) and perform any necessary migrations.</p>
 
 <p>If <code>--merge</code> is specified then <code>git merge</code> is used to include updates
 (rather than <code>git rebase</code>).</p>

manpages/brew.1

@@ -542,7 +542,7 @@ Remove a tapped repository\.
 .
 .TP
 \fBupdate\fR [\fB\-\-merge\fR] [\fB\-\-force\fR]
-Fetch the newest version of Homebrew and all formulae from GitHub using \fBgit\fR(1)\.
+Fetch the newest version of Homebrew and all formulae from GitHub using \fBgit\fR(1) and perform any necessary migrations\.
 .
 .IP
 If \fB\-\-merge\fR is specified then \fBgit merge\fR is used to include updates (rather than \fBgit rebase\fR)\.