* Add {wakefield} as dependency for Strings chapter
* Move footnote into body of text
The footnote appears to be redundant with the more vague paragraph
immediately following it in the main body of the text, so combine their
information instead.
* Make explicit that `coalesce()` replaces NAs
* Fix definition of `start` & `end` for `str_sub()`
* Edit section on Letter variations
* Edit section on Locale-dependent function
* Apply suggestions from code review
Co-authored-by: Mine Cetinkaya-Rundel <cetinkaya.mine@gmail.com>
Co-authored-by: Mine Cetinkaya-Rundel <cetinkaya.mine@gmail.com>
