Finish minimal documentation for all modules

[matthewhammer]

Finish writing at least one line of description for each method in the module or class below:

• [x] Buf (rename to Buffer?)
• [x] Array -- https://github.com/dfinity/motoko-base/pull/138
• [x] Deque -- https://github.com/dfinity/motoko-base/pull/132
• [x] Hashmap
• [x] RBTree -- https://github.com/dfinity/motoko-base/pull/140
• [x] Trie -- https://github.com/dfinity/motoko-base/pull/130
• [x] TrieMap -- https://github.com/dfinity/motoko-base/pull/128
• [x] TrieSet -- https://github.com/dfinity/motoko-base/pull/139
• [x] AssocList -- https://github.com/dfinity/motoko-base/pull/141

cc @crusso @kritzcreek @rossberg

[kritzcreek]

HashMap done in #121

[crusso]

@matthewhammer Any progress here? Would help out but have other fires to put out...

[matthewhammer]

@crusso I have time today for this. I'll get to it all this week. Sorry for the delays.

[matthewhammer]

(Finally) made a PR for this.

I started with TrieMap https://github.com/dfinity/motoko-base/pull/128 to get used to the workflow and it seems very nice. I have one question, which I put there in the PR.

Seems easy enough to finish the rest ~today~ this week.

[matthewhammer]

Update: PRs are open for each module in the list, but many need a review.