[KishSiva]

Fixes #4869

Chapter 6 teaches let...else (and the table of contents names it), but three places that enumerate where patterns or else appear still leave it out. This adds it to each of them:

• src/ch06-00-enums.md: the chapter overview now ends with "the if let and let...else constructs" instead of only if let.
• src/appendix-01-keywords.md: the else entry now reads "Fallback for if, if let, and let...else control flow constructs".
• src/ch19-01-all-the-places-for-patterns.md: the sentence "Prior to this chapter, we had only explicitly discussed using patterns with match and if let" now also names let...else and while let (both were discussed in earlier chapters), and there is a new short "let...else Statements" subsection after the if let one. It describes what let...else binds, that the else block must diverge, and that it accepts a refutable pattern, which hands off to the next section on refutability. The example is a three-line rust,ignore block, so no listing directory is added.

Prose is wrapped at 80 columns like the surrounding text. Nothing under nostarch/ is touched.