Added a few readability and consistency edits to sessions

* $_SESSION is more-so of a proper noun since it is a predefined variable in PHP, therefore doesn't necessitate a "the."
* Compressed the first two sentences under "Session Configuration."  I felt that it read weird when both sentences started with the same phrase.
* Missing space in user agent.
* Missing space between "sessions" and "ids."
* Edited the first "Reading & writing session data" sentence to remove a shared "in" between "Depending on the context you are" and "your application you have different classes."