docs: Explicitly state that the 'Fixes:' tag shouldn't split lines

docs: Explicitly state that the 'Fixes:' tag shouldn't split lines

...and use a commit with an obnoxiously long summary in the example to
make it abundantly clear that keeping the tag on a single line takes
priority over wrapping at 75 columns.  Without the explicit exemption,
one might assume splitting the tag is acceptable, even encouraged, e.g.
due to being conditioned by checkpatch's line length warning.

Per Stephen's scripts[1] and implied by commit 1b08c3185588 ("checkpatch:
avoid some commit message long line warnings"), splitting the 'Fixes:'
tag across multiple lines is a no-no, presumably because parsing multi-
line tags is unnecessarily painful.

[1] https://lkml.kernel.org/r/20190216183433.71b7cfa7@canb.auug.org.au

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>