Squashing of 10 commits

Heading style suggestion:
Fix overlapping (e.g. http://tarantool.org/doc/getting_started.html#starting-another-tarantool-instance-and-connecting-remotely)
Proper margins

Codeblock style suggestion:
Add greyish background for inline code
Add background for bash codeblocks
Add overflow property for bash codeblocks:

Add horizontal scrolling for text in code-blocks

Fix multiple issues listed in #1033 (Database section):
Addressed style inconsistency
Fixed module descriptions
Added hotlinks for functions
Minor style additions

Fix formatting issues in Reference Manual

Fix Lua code highlighting

A rather blunt fix that offers a temporary workaround.
The reason behind original Lua lexer not highlighting Lua code is '!' used as
delimiter. Pygments fails to parse '!' as a Lua code entity and tells Sphinx to
leave the code unhighlighted. The fix adds custom Lua lexer by alias 'lua_tarantool',
with '!' added as a punctuation token (line 60 in LuaLexer.py). A proper fix would
probably be to extend the lexer with all Tarantool extensions over standard Lua modules.
Lua code block directive now should be used like this:

.. code-block:: lua_tarantool

	lua code

Fix remaining problems from #1033

Review styles for Terminal session and inline code

Improve formatting for C API

Improve formatting in Lua tutorial