DOC: streamline http-reuse and connection naming definition

DOC: streamline http-reuse and connection naming definition

With the introduction of "pool-conn-name", documentation related to
http-reuse was rendered more complex than already, notably with multiple
cross-references between "pool-conn-name" and "sni" server keywords.

Took the opportunity to improve all http-reuse related documentation.
First, "http-reuse" keyword general purpose has been greatly expanded
and reordered.

Then, "pool-conn-name" and "sni" have been clarified, in particular the
relation between them, with the foremost being an advanced usage to the
default SSL SNI case in the context of http-reuse. Also update
attach-srv rule documentation as its name parameter is directly linked
to both "pool-conn-name" and "sni".