Fix docstrings `reformat` warning

[rbonichon]

In reaction to the warning below , this PR removes docstring in implementation files. In addition, some are moved to the interface files, where I think they clearly belong

dune exec --profile=dev src/app/reformat/reformat.exe -- -path . -check
(monitor.ml.Error
 ("Process.run failed"
  ((prog ocamlformat)
   (args
    (--doc-comments=before ./src/lib/merkle_mask/masking_merkle_tree.ml))
   (exit_status (Exit_non_zero 1)) (stdout "")
   (stderr
    ("ocamlformat: ignoring \"./src/lib/merkle_mask/masking_merkle_tree.ml\" (misplaced documentation comments - warning 50)"
     "File \"./src/lib/merkle_mask/masking_merkle_tree.ml\", lines 55-73, characters 2-23:"
     "55 | ..(** Structure managing cache accumulated since the \"base\" ledger."
     "56 | "
     "57 |     Its purpose is to optimize lookups through a few consequitive masks"
     "58 |     (by using just one map lookup instead of [O(number of masks)] map lookups)."
     "59 | " ... "70 | "
     "71 |     Garbage-collection/rotation mechanism for [next] and [current] is based on idea to set"
     "72 |     [current] to [next] and [next] to [t.maps] when the mask at which accumulation of [next] started"
     "73 |     became detached. *)"
     "Warning 50 [unexpected-docstring]: ambiguous documentation comment"
     "Hint: (Warning 50) This file contains a documentation comment (** ... *) that the OCaml compiler does not know how to attach to the AST. OCamlformat does not support these cases. You can find more information at: https://github.com/ocaml-ppx/ocamlformat#overview. If you'd like to disable this check and let ocamlformat make a choice (though it might not be consistent with the ocaml compilers and odoc), you can set the --no-comment-check option."
     ""))))

[rbonichon]

!ci-build-me

[rbonichon]

!ci-build-me