doc: document the fds behind stdin/out/err

Its common knowledge on unix, but node documentation depends on knowing
this, as it exposes both streams named after stdio, and the fd numbers,
so make this explicit.

Closes #8624
doc: remove wrong spawn error checking example

The example is not correct, and hasn't been since at least v0.10. Exec
errors are emitted as an error event, and documented as such in the
'error' event section.
doc: remove wrong note about spawn env

env was documented as not being process.env under some condtions, which
is no longer correct.
doc: spawn cwd default is current working dir

Default was perhaps obvious, but not documented.
doc: hyperlink and re-order spawn documentation

The lack of hyperlinks requires scrolling past pages of examples to find
the options documentation, the default value of the 'stdio' option was
not documented, and the documentation was not ordered readably.