manpage: document '--dry-run' for 'prune' command

Library/Homebrew/manpages/brew.1.md

@@ -343,10 +343,13 @@ With `--verbose` or `-v`, many commands print extra debugging information. Note
     Pin the specified <formulae>, preventing them from being upgraded when
     issuing the `brew upgrade` command. See also `unpin`.
 
-  * `prune`:
+  * `prune` [`--dry-run`]:
     Remove dead symlinks from the Homebrew prefix. This is generally not
     needed, but can be useful when doing DIY installations.
 
+    If `--dry-run` or `-n` is passed, show what would be removed, but do not
+    actually remove anything.
+
   * `reinstall` <formula>:
     Uninstall then install <formula>
 

share/doc/homebrew/brew.1.html

@@ -271,8 +271,11 @@ precedence over <code>--verbose</code>).</p>
 valid version is <code>v1</code>.</p></dd>
 <dt><code>pin</code> <var>formulae</var></dt><dd><p>Pin the specified <var>formulae</var>, preventing them from being upgraded when
 issuing the <code>brew upgrade</code> command. See also <code>unpin</code>.</p></dd>
-<dt class="flush"><code>prune</code></dt><dd><p>Remove dead symlinks from the Homebrew prefix. This is generally not
-needed, but can be useful when doing DIY installations.</p></dd>
+<dt><code>prune</code> [<code>--dry-run</code>]</dt><dd><p>Remove dead symlinks from the Homebrew prefix. This is generally not
+needed, but can be useful when doing DIY installations.</p>
+
+<p>If <code>--dry-run</code> or <code>-n</code> is passed, show what would be removed, but do not
+actually remove anything.</p></dd>
 <dt><code>reinstall</code> <var>formula</var></dt><dd><p>Uninstall then install <var>formula</var></p></dd>
 <dt><code>rm</code>, <code>remove</code>, <code>uninstall</code> [<code>--force</code>] <var>formula</var></dt><dd><p>Uninstall <var>formula</var>.</p>
 

share/man/man1/brew.1

@@ -367,9 +367,12 @@ If \fB\-\-json=\fR\fIversion\fR is passed, the output will be in JSON format\. T
 Pin the specified \fIformulae\fR, preventing them from being upgraded when issuing the \fBbrew upgrade\fR command\. See also \fBunpin\fR\.
 .
 .TP
-\fBprune\fR
+\fBprune\fR [\fB\-\-dry\-run\fR]
 Remove dead symlinks from the Homebrew prefix\. This is generally not needed, but can be useful when doing DIY installations\.
 .
+.IP
+If \fB\-\-dry\-run\fR or \fB\-n\fR is passed, show what would be removed, but do not actually remove anything\.
+.
 .TP
 \fBreinstall\fR \fIformula\fR
 Uninstall then install \fIformula\fR