Add chapter on Attributes in language section.

[beberlei]

I am not particularly good at documneting things, but this is an acceptable start I hope, for the Attributes feature. it covers the basics:

• Syntax
• Reflection API
• Declaring Attribute Classes

A bunch of things are missing for now:

• Better Examples, not Thing and such
• ReflectionAttribute class in reflection book
• getArguments() methods in every relevant reflection component
• Explanation of internal attributes declared in Core and Extensions

after merging this, doc-base needs a small patch to render this:

diff --git a/manual.xml.in b/manual.xml.in
index 6e59239d..6c8b1ba8 100644
--- a/manual.xml.in
+++ b/manual.xml.in
@@ -80,6 +80,7 @@
   &language.errors;
   &language.exceptions;
   &language.generators;
+  &language.attributes;
   &language.references;
   &language.predefined.variables;
   &language.predefined.exceptions;

[Girgias]

Can't comment on the general text as I don't really understand the feature, but the usage of you should be avoided in the docs. So if you could change that that would be great. :)

[Girgias]

Should we add links to the relevant pages?

[cmb69]

Might be more noise than actually being useful.

[cmb69]

Thank you! Besides the few nits, I think this is good to be merged. The rest of the documentation updates can be done later.