docs: fix inaccurate comments in internal/rpn/variables.go

- Load: correct interface and method docs to reflect that all existing
  variables are replaced (not merged/overwritten)
- Load: remove misleading 'thread-safe for concurrent reads' claim; the
  method acquires a write lock, not a read lock
- Save: clarify that it uses a read lock so it doesn't block concurrent
  readers, rather than the confusing 'concurrent writes' claim

0 files changed, 0 insertions, 0 deletions