This patch uniformizes warning messages. I used the most common style
already present in the code base:
* Capitalize the first word.
* End the message with a full stop.
* "Rails 5" instead of "Rails 5.0".
* Backticks for method names and inline code.
Also, converted a few long strings into the new heredoc convention.
The current style for warning messages without newlines uses
concatenation of string literals with manual trailing spaces
where needed.
Heredocs have better readability, and with `squish` we can still
produce a single line.
This is a similar use case to the one that motivated defining
`strip_heredoc`, heredocs are super clean.
