doc: reword stream docs to clarify that decodeStrings encodes strings

I was implementing a Writable stream and misunderstood `decodeStrings`
to mean 'will decode `Buffer`s into `string`s before calling `_write`'.
This change adds a little more detail to the description of
`decodeStrings` to clarify its effect on a Writable stream & what gets
passed to `_write`.

Changing the name of the option to `encodeStrings` would make it much
easier to understand, but the name was chosen in 2012 and the option
used in many projects (22k mentions of 'decodeStringsr in JS projects in
GitHub). Deprecating the old name & rolling out a replacement is beyond
my capabilities as a first-time contributor.

PR-URL: https://github.com/nodejs/node/pull/25468
Fixes: https://github.com/nodejs/node/issues/25464


Reviewed-By: Anna Henningsen <anna@addaleax.net>