Wrap code snippets in +, not backticks, in sdoc

I grepped the source code for code snippets wrapped in backticks in the comments
and replaced the backticks with plus signs so they are correctly displayed in
the Rails documentation.

[ci skip]
Merge pull request #17695 from claudiob/replace-all-backticks-with-pluses

Wrap code snippets in +, not backticks, in sdoc [ci skip]