doc: added Buffer.from(string) to functions that use buffer pool

[Whitecx]

Buffer.from(string) is one of the functions that may use the pre-allocated buffer. It's mentioned in the description of Buffer.from(array), but not in Buffer.from(string), or in the two other places where functions that behave this way are listed, so this commit adds those references.

[jasnell]

There appear to be a number of additional commits here that are unrelated to the doc change. Can I ask you to remove the extra commits?

[Whitecx]

There appear to be a number of additional commits here that are unrelated to the doc change. Can I ask you to remove the extra commits?

Sorry about that, everything should be cleaned up now.

[Whitecx]

@jasnell Does this change feel substantive? I wanted to make this PR when I had some time, because I'd used buffer.from(string) and was confused as to why the underlying buffer had other data in it. When I didn't see any explanation in the buffer.from(string) section of the docs, I actually went to the source code and found where the buffer pool was used when creating strings long before I noticed that this behavior was documented at the bottom of buffer.from(array) 😂.

It would've saved me some time had it been in the buffer.from(string) section of the docs, so I thought it might help others as well.