Update clarifying notes in filter.md (#2670)

Author: mercury233

website/docs/filter.md

@@ -144,7 +144,7 @@ filter {}
 
 * Filters can be viewed as a scoping concept.  A currently set filter goes 'out of scope' when either a filter reset operation is invoked or a project definition is started.
 
-* Filters are whitespace sensitive. For example, a filter of `system:not windows` is fundamentally different from `system: not windows`.
+* Filters are whitespace sensitive. For example, a filter of `system:not windows` is fundamentally different from `system: not windows`. The former means `system ~= "windows"`, whereas the latter will attempt to match `system == " not windows"`.
 
 ### See Also ###