Update RELEASE_NOTES.md for PR #982 (improved named snippet error message) (#991)

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RELEASE_NOTES.md

@@ -2,6 +2,9 @@
 
 ## [Unreleased]
 
+### Fixed
+* Improve error message when a named code snippet is not found (e.g. `(*** include:name ***)` with undefined name now reports the missing name clearly). [#982](https://github.com/fsprojects/FSharp.Formatting/pull/982)
+
 ### Changed
 * Update FCS to 43.10.100. [#935](https://github.com/fsprojects/FSharp.Formatting/pull/966)