Improve manual for 1.6

[pdmef]

The manual still has lots and lots of inconsistencies in style and markup (e.g. markup for function names). These should be revisited to make it ready for 1.6. If possible, we should also add a section on typographic conventions (if not, we should define them in this ticket to fix inconsistencies).

[Rocco Rutte <pdmef@…>]

(In [6fd1fa2d98bc]) Manual: fix/improve markup/content, see #3109

[vinc17]

There are errors in doc/manual.xml.head:

• eiter -> either
• won't be send -> won't be sent
• without having to needing -> without having to need
• if does exist already -> if it does exist already

And shouldn't RFC822 be replaced by RFC2822?

In the "Pattern modifiers" section, there are inconsistencies between the singular and the plural (e.g. ~b vs ~e).

Also, there's something missing here:

<varlistentry>
<term></term>
<listitem><para></para></listitem>
</varlistentry>

[Rocco Rutte <pdmef@…>]

(In [a2fc5e7c77ae]) Manual: Fix style and typos. Noted by Vincent Lefevre, see #3109.

[pdmef]

It would be really nice if we could find an english native speaker going over the manual and fixing grammar and/or punctuation before 1.6 and after all changes are made.

[pdmef]

Simple and clean CSS stylesheet for manual