Document @leaf

[Xophmeister]

Is your feature request related to a problem? Please describe.
The @leaf capture group is a rather important concept, but it is not documented at all in the README.

Describe the solution you'd like
Document @leaf, explaining what it's for and the consequences it has on formatting.

Additional context

• Note that while @leaf should mark notes as immutable to formatting, @delete still takes precedence.
• See also discussion in Add support for @upper_case @lower_case captures #838