docs: add release note for addinvoice msat amount

Author: GeorgeTsagk

docs/release-notes/release-notes-0.14.2.md

@@ -78,6 +78,8 @@
 * All Taproot Asset Channel related commands (`litcli ln ...`) can now [use
   a new `--group_key` flag to interact with grouped asset
   channels](https://github.com/lightninglabs/lightning-terminal/pull/1052).
+* The `litcli ln addinvoice` command now supports adding an invoice with a fixed
+  msat amount.
 
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