Update Install.rst

Line 40 of Install.rst, removed additional wording "and is the one where the software is more tested" as it isn't necessary
Line 95 of Install.rst, removed "Take" from "Take special attention..." and replaced with "Pay special attention.." as a standard usage of the term.
Line 104 of Install.rst, changed "doesn't" to "won't" to reduce duplicate word usage in the sentence.
Update Advanced-Install.rst

Line 14, added a space between 'unit' and 'tests.'
Line 17, removed period after 'aka' as it's not the end of the sentence.
Line 29, Added period between ...please report it back to us too. We're interested... to avoid run-on sentence.
Update update.rst

Line 7, added the word version to clarify what is being updated. The w3af version is being updated.
Line 16, added comma after update for grammar correction.  
Line 44, added quotes around the last ''feature'' entry to make it highlighted as the other instances of the branches. removed 'which' from ...disturb other users which require... and replace with 'that' for grammar.
Merge pull request #2021 from jarrodcoulter/develop

Improved documentation by @jarrodcoulter