doc: fix typos

[NathanBaulch]

Just thought I'd contribute some typo fixes that I stumbled on. Nothing controversial (hopefully).

Use the following command to get a quick summary of the specific corrections made:

git diff HEAD^! --word-diff-regex='\w+' -U0 \
  | grep -E '\[\-.*\-\]\{\+.*\+\}' \
  | sed -r 's/.*\[\-(.*)\-\]\{\+(.*)\+\}.*/\1 \2/' \
  | sort | uniq -c | sort -n

FWIW, the top typos are:

• udpate (9)
• flakyness (8)
• protoype (6)
• uneeded (6)
• transferrable (5)
• suport (4)
• clearify (4)
• prototoype (4)
• unecessary (4)
• execption (3)
• existance (3)
• sypnosis (3)
• comparions (3)
• explicity (3)
• sript (3)
• iin (3)
• interuption (3)
• unnecesary (3)