Merge branch 'ta/hash-function-transition-doc'

Update formatting and grammar of the hash transition plan
documentation, plus some updates.

* ta/hash-function-transition-doc:
  doc: use https links
  doc hash-function-transition: move rationale upwards
  doc hash-function-transition: fix incomplete sentence
  doc hash-function-transition: use upper case consistently
  doc hash-function-transition: use SHA-1 and SHA-256 consistently
  doc hash-function-transition: fix asciidoc output

Author: gitster