Avoid spurious comment highlighting

[dandersson]

The previous comment highlighting incorrectly highlighted the line following # characters occurring within a cell/token as comments, in cases such as foo#bar and foo #bar.

The #bar strings above should not be highlighted as comments, as they do not describe the start of a cell or token, which are delimited by at least either two spaces or a tab.

http://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#rules-for-parsing-the-data

I removed the note on multiline comments, as they are not supported. See e.g.

• https://github.com/robotframework/robotframework/issues/1825
• https://groups.google.com/forum/#!topic/robotframework-users/yjgeHIcHfTs
• https://stackoverflow.com/q/26129435