doc: clarified stream.Transform callback argument logic

[rafasofizada]

Clarified that stream.Transform callback second argument is passed only if the first argument is null, i.e. no error occured processing the chunk.

From the way the documentation is written right now, I expected the second argument to be forwarded to transform.push(), even if the first argument is an Error. I was implementing a truncation stream, and passed both an Error ("stream exceeded maximum size"; could be optionally caught and ignored), and the last (truncated) chunk. Only after digging through the relevant implementaton I could confirm for sure that the second argument is entirely ignored if the first one is not null.

Please tell me if you want to adjust the wording somehow, or whether I should add an alternative example to the one following the line "In other words, the following are equivalent:". Hopefully you'll find this useful.

[lpinca]

Suggested change

	`transform.push()` method, but only if the first argument is `null`. In other
	`transform.push()` method, but only if the first argument is falsy. In other

[rafasofizada]

@lpinca thanks for the approval! Could you please advice how would I go about editing the commit message to pass the lint-commit-message check?

[lpinca]

Wrap the commit message body to 72 chars per line. Also use an imperative verb for commit message title (clarified -> clarify).

doc: clarify transform._transform() callback argument logic

Clarify that `transform._transform()` callback second argument is
used only if the first argument is `null`, i.e. no error occured
processing the chunk.

[rafasofizada]

@lpinca I tried to amend the commit message and force-push it, but apparently messed something up. Opened a new PR with the proposed changes and fixed commit message: #48680