[doc] Describe available client options.
[doc] Fix code snippet used for client options.
[doc] Add note about client options values returned by callables.

Predis will not attempt to use its standard configuration path for
options when using callables as initializers, this makes sense and
it is an intended behavior to give users more flexibility over the
configuration of the client but it can be confusing and unexpected
in certain cases such as with the combination of server profile and
key prefix.
Merge pull request #97 from nrk/doc-client-options

[doc] Describe available client options.