docs: improve "rule" definition (#364)

This wasn't really incorrect, but it was a bit tough to understand, IMO.

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Signed-off-by: Todd Baert <todd.baert@dynatrace.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: toddbaert

specification/glossary.md

@@ -203,7 +203,7 @@ Pseudorandomly resolve flag values using a context property, such as a targeting
 
 ### Rule
 
-A rule is some criteria that's used to determine which variant a particular context should be mapped to.
+A rule is some criteria or logic used to assign a variant during an evaluation.
 
 ## SDK Paradigms