Update Python hashbang to respect environment

The /usr/bin/python hashbang causes doc builds to fail when Python3 is
the default Python and Python2 is being used from within a virtualenv or
Pythonbrew environment.

Signed-off-by: Sharif Olorin <sio@tesser.org>
Merge pull request #1438 from fghaas/doc-fix

doc: Add "nearest power of two" to PG rule-of-thumb

Reviewed-by: Sage Weil <sage@inktank.com>
Merge pull request #1423 from fractalcat/respect-python-env

Update Python hashbang to respect environment
doc/release-notes: note that WATCH can get ENOENT now

Signed-off-by: Sage Weil <sage@inktank.com>
mailmap: Danny Al-Gaaf name normalization

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
doc: fix formatting on PG recommendation

Previous commit (047287afbe0ddfaaafd05e9dbf25c1c7dea9a1be) broke
formatting on the formula, and also made mixed formula and text oddly,
which on second thought didn't look too good.

Add the note about the power of two to the following paragraph
instead, in prose.

Signed-off-by: Florian Haas <florian@hastexo.com>