RFC cleanup

RFC update  to reflect the actual flags/semantics

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: mbachorik

extensions/RFC-EXTENSION-SYSTEM.md

@@ -1203,10 +1203,8 @@ Next steps:
 
 **Options:**
 
-- `--from URL`: Install from custom URL or Git repo
-- `--version VERSION`: Install specific version
-- `--dev PATH`: Install from local path (development mode)
-- `--no-register`: Skip command registration (manual setup)
+- `--from URL`: Install from a remote URL (archive). Does not accept Git repositories directly.
+- `--dev`: Install from a local path in development mode (the PATH is the positional `extension` argument).
 - `--priority NUMBER`: Set resolution priority (lower = higher precedence, default 10)
 
 #### `specify extension remove NAME`