No documentation on loading state & their lock leads to read-only access to the opam 2.2 state fails when opam-state 2.1.* is used

[kit-ty-kate]

#=== ERROR while compiling opam-dune-lint.0.2 =================================#
# context              2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.4.14.1 | file:///home/opam/opam-repository
# path                 ~/.opam/4.14/.opam-switch/build/opam-dune-lint.0.2
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p opam-dune-lint -j 255 @install @runtest
# exit-code            1
# env-file             ~/.opam/log/opam-dune-lint-7-c1162b.env
# output-file          ~/.opam/log/opam-dune-lint-7-c1162b.out
### output ###
# File "tests/test_vendoring.t", line 1, characters 0-0:
# File "tests/test_empty_dune.t", line 1, characters 0-0:
# File "tests/test_dune.t", line 1, characters 0-0:
#          git (internal) (exit 1)
# (cd _build/.sandbox/3887dc4e8b1d66b4fd37e214ba236a87/default && /usr/bin/git --no-pager diff --no-index --color=always -u ../../../default/tests/test_dune.t tests/test_dune.t.corrected)
# diff --git a/../../../default/tests/test_dune.t b/tests/test_dune.t.corrected
# index 35227f0..c5e6ec7 100644
# --- a/../../../default/tests/test_dune.t
# +++ b/tests/test_dune.t.corrected
# @@ -33,23 +33,20 @@ Replace all version numbers with "1.0" to get predictable output.
#  Check that the missing libraries are detected:
#  
#    $ opam-dune-lint </dev/null
# -  test.opam: changes needed:
# -    "fmt" {>= "1.0"}                         [from /]
# -    "bos" {with-test & >= "1.0"}             [from /]
# -    "opam-state" {with-test & >= "1.0"}      [from /]
# -  Note: version numbers are just suggestions based on the currently installed version.
# -  Run with -f to apply changes in non-interactive mode.
# -  [1]
# +  [ERROR] Refusing write access to /home/opam/.opam, which is more recent than this version of opam (2.2~alpha > 2.1), aborting.
# +  opam-dune-lint: internal error, uncaught exception:
# +                  OpamStd.OpamSys.Exit(15)
# +                  
# +  [125]

According to @rjbou this is unintended behaviour and using 2.1 library with a 2.2 root should work as long as it is in read-only mode.
Looking into the source of opam-dune-lint.0.2, only one lock access to opam root can be found and it is indeed read-only:

$ opam source opam-dune-lint.0.2
$ cd opam-dune-lint.0.2
$ grep -R Lock
index.ml:  OpamGlobalState.with_ `Lock_none @@ fun gt ->

[rjbou]

the issue is mainly in opam-dune-lint, and opam api documentation. With the patch applied in ocurrent/opam-dune-lint#36 (ie specifying which lock is required), there is write issue. To preserve unwanted writes, opam takes a write lock by default.
There is no documentation on that behaviour, we need to update it.