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 bf4daf12a9 ("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>

Documentation/process/submitting-patches.rst

@@ -182,9 +182,11 @@ change five years from now.
 
 If your patch fixes a bug in a specific commit, e.g. you found an issue using
 ``git bisect``, please use the 'Fixes:' tag with the first 12 characters of
-the SHA-1 ID, and the one line summary.  For example::
+the SHA-1 ID, and the one line summary.  Do not split the tag across multiple
+lines, tags are exempt from the "wrap at 75 columns" rule in order to simplify
+parsing scripts.  For example::
 
-	Fixes: e21d2170f366 ("video: remove unnecessary platform_set_drvdata()")
+	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
 
 The following ``git config`` settings can be used to add a pretty format for
 outputting the above style in the ``git log`` or ``git show`` commands::