Polish groovy.lang package documentation

[marcwrobel]

Fix typos and improve documentation (javadoc, comments) in the groovy.lang package.

Note that the following words / expressions were preferred :

• return back -> returns (when possible because it is easier to read),
• multi-threaded -> multithreaded (more common, see https://en.wikipedia.org/wiki/Multithreading)
• whether or not -> whether (when possible because it is easier to read),
• meta class -> metaclass (incorrect, see https://en.wikipedia.org/wiki/Metaclass).

Trailing whitespaces were also removed in the process.

[paulk-asert]

Merged, thanks!