doc: drop 50-character limit for commit messages
Replace the current limit of 50 characters for the first line of the commit message with a more flexible formulation.
Copying the reasoning from https://github.com/nodejs/node/pull/8294#issuecomment-242902382:
50 characters is a pretty hard limit when you need to include something like child_process:
and/or are a non-native speaker with reduced vocabulary.