doc/ref: propose reserving identifiiers starting with `__`
doc/ref: propose reserving identifiiers starting with `__`

Change-Id: I9e70fd765de6f448dbc0fa2cd1f3e78024151fd2
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1782
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@google.com>

cuelang/cue