CURLOPT_POSTFIELDS.3: Clarify what happens when set empty

When CURLOPT_POSTFIELDS is set to an empty string libcurl will send a
zero-byte POST. Prior to this change it was documented as sending data
from the read callback.

This also changes the wording of what happens when empty or NULL so that
it's hopefully easier to understand for people whose primary language
isn't English.

Bug: https://github.com/curl/curl/issues/862
Reported-by: Askar Safin