Update monolog.rst

Changed \n => &#xA; in xml example.
Added documentation for buttons in forms
[Book] [Security] Fix typo
Adding the mapped type to the form type references
[Reference][Forms] Fix options type for hours, minutes, seconds
[Reference][Forms] Fix typo introduced in previous commit
Previous session options to security configuration
Add documentation for option to set require_previous_session in form
login.
Add space after #
Added limitations in 'Managing Common Dependencies with Parent Services' with a caution
Fixed spelling and grammar errors
Updated contributor branch references - 2.0 no longer maintained

The current documentation refers throughout to making bugfix and
corrections against the 2.0 branch, however this is no longer
maintained. Updated with references to 2.1 to make this more
obvious to new contributors.
[Process] Update use of Process constants
[Process] Add documentation about `signal` and `getPid` methods
document the doctrine compiler pass
Typo fixed
cs cleanup
Improving Web server configuration
kernel.bundle_dirs doesn't exist in KernelParameters
Documented that the Optional and Required constraints were moved to the Symfony\Component\Validator\Constraints\ namespace
adding doc for mongodb