Missing documentation for `ast-plus` tracking mode

[WojciechMazur]

The ast-plus tracking mode added in #1459 is missing documentation. There is not mention in the docs about the behaviour of this mode

[FrankPortman]

I would like to add a secondary request to this - #1493 seems no longer relevant for version 7 + bzlmod. I would like to use ast-plus and have everything working besides non hermeticity introduced by:

Rule 'rules_scala++scala_deps+scala_compiler_source_2_13_16' indicated that a canonical reproducible form can be obtained by modifying arguments integrity = "sha256-6SLcL5fVRf3Ag/L88pFesAx2R/N3C5VIneC8nve1Sj4="