Add guidance on using colons in bullet lists, and make styleguide adhere closer to styleguide.
What does this MR do?
Makes explicit a style rule that has been implicitly followed for a long time.
Note use of capital letter after :
. Are we happy with that, @gl-docsteam?
Also fixes some places where the style guide doesn't follow the style guide.
Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/51086.