Desugaring doc comments

[mattheww]

#270 added descriptions of the process of desugaring doc comments to doc attributes to chapter 2 (lexical elements), defining the notion of an equivalent attribute doc, and to various places in chapter 20 (Macros), using that notion.

The descriptions in chapter 2 describe this process for both inner and outer doc comments, but the descriptions in chapter 20 describe it only for outer doc comments. I think the descriptions in chapter 20 should say it applies to inner doc comments too.

Those descriptions are in:

• §20.4:3 (fls_40xq8Ri1OMZZ)
• §20.4:13 (fls_nNrs4EC3ff5T)
• §20.4:23 (fls_vqIZaEl4EKu5)
• §20.4:29 (fls_AJmPrhHfZo6J)

(Also there's a typo "amd" for "and" in §2.5:16, fls_ok0zvo9vcmzo.)