Enable warn(missing_docs) for all component crates

[sffc]

All ICU component crates should be fully documented. To track this, we should enable warn(missing_docs) as suggested in boilerplate.md.

Pre-existing sub-issues:

• https://github.com/unicode-org/icu4x/issues/1025
• https://github.com/unicode-org/icu4x/issues/1028
• https://github.com/unicode-org/icu4x/issues/1029

This issue does not track utils or experimental crates.

[echeran]

component	status
calendar	complete
collator	complete
collections	complete
datetime	complete
decimal	complete (#1025)
icu	complete
list	complete
locale_transform	complete
locid	complete (#1028)
normalizer	complete
plurals	complete (#2108)
properties	complete
timezone	complete