doc: slightly relax 50 character rule

Allow commit message first line to exceed 50 chars if necessary

PR-URL: #16523
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>

Author: jasnell

CONTRIBUTING.md

@@ -309,8 +309,8 @@ notes about [commit squashing](#commit-squashing)).
 A good commit message should describe what changed and why.
 
 1. The first line should:
-   - contain a short description of the change
-   - be 50 characters or less
+   - contain a short description of the change (preferably 50 characters or less,
+     and no more than 72 characters)
    - be entirely in lowercase with the exception of proper nouns, acronyms, and
    the words that refer to code, like function/variable names
    - be prefixed with the name of the changed subsystem and start with an