mu4e: handle conversion from pcre regexps

mu server uses PCRE-compatible regular expressions, but mu4e needs
Emacs-style. In simple cases, the two are the same, but when it gets
slightly more complicated they do not.

E.g.,
  (foo|bar) => \(foo\|bar\)

Mu4e can do the conversion automatically, but for this the pcre2el
package (available in MELPA) is required, and is used if user installs
is. Anyway, this code implements the automatic conversion and adds some
documentation.

Fixes #2816.

mu4e/mu4e.texi

@@ -403,9 +403,8 @@ You can add some e-mail addresses, so @t{mu} recognizes them as yours:
 indexing messages. If you want to change them, you need to @t{init}
 once again.
 
-The addresses may also be basic PCRE regular expressions, wrapped in
-slashes, for example:
-
+The addresses may also be basic PCRE regular expressions, wrapped in slashes,
+for example:
 @example
   $ mu init --maildir=~/Maildir '--my-address=/foo-.*@@example\.com/'
 @end example
@@ -413,6 +412,19 @@ slashes, for example:
 If you want to see the values for your message-store, you can use
 @command{mu info}.
 
+Note: unfortunately, PCRE regular expressions are not the same as Emacs regular
+expressions. Very simple ones match, but e.g. @t{(foo|bar)} in PCRE syntax is
+specified as @t{\(foo\|bar\)} in Emacs/@t{mu4e}.
+
+The good news is that @t{mu4e} can do the conversion automatically; however for
+this to work, users needs to install the @t{pcre2el} package (available in
+MELPA), and ensure it is available in @t{load-path} when Emacs runs (a normal
+package-installation takes care of this). After that, @t{mu4e} takes care of the
+translation automatically.
+
+It is possible to come up with complicated PCRE regular expressions that are not
+translatable; if necessary, experiment with @t{pcre2el} to avoid this.
+
 @node Indexing your messages
 @section Indexing your messages