libcore/pin: Minor grammar corrections for module documentation

This is by no means exhaustive, but I noticed a few grammatical errors
when reading the documentation, and decided just to push these.

Some standard rules/guidelines I followed:

 * Do not split infinitives, ie "not to move" instead of "to not move"

 * Do not use "since" when you want to say "because" or "as" - the word
   "since" has a temporal meaning

In addition:

 * Fix a small typo: "Similarily" should be "Similarly"

 * Delete double-spaces after full stop

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