doc: fix exec stdout & stderr default type in child_process

[sartrey]

• v4.4.4:
• Darwin:

There is an inaccurate line in child_process documents.
** origin text **
If encoding is 'buffer', Buffer objects will be passed to
the callback instead

Actually, if encoding parameter can not be identified as encoding (Buffer.isEncoding)
exec will also output buffer rather than string

Could I provide a pr to fix the document?

[cjihrig]

Could I provide a pr to fix the document?

Sure.

[cryptoquick]

I hope we are aware this is the most evilest node.js issue. 😈