Avoid spurious comment highlighting

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>

after/syntax/robot.vim

@@ -74,8 +74,11 @@ syn match builtInLibrary            "\c\<\(Wait Until Keyword Succeeds\|Variable
 "------------------------------------------------------------------------
 " Regions
 "------------------------------------------------------------------------
-" Single-line comments. Are there multi-line comments?
-syn region robotComment     display start="#" excludenl end="$"
+" Comments are defined as: "All characters following the hash character (#),
+" when it is the first character of a cell". The start of a cell is either the
+" first non-whitespace content on a line or delimited either by two spaces or
+" a tab character.
+syn region robotComment     display start="\(^\|  \|\t\)[ \t]*#" excludenl end="$"
 syn region robotString      start="\"" excludenl end="\""
 
 "------------------------------------------------------------------------