kernel: Fix code:get_doc/1,2 when cover_compiled

[williamthome]

This PR fixes a bug getting docs when a module is compiled by the cover module. Also, the cover_compiled error was missing in the spec.

To reproduce, first compile the module via cover:compile and run code:get_doc.

Find this bug via the new doctest, for example:

% src/otp_doctest
-module(otp_doctest).

-export([foo/0]).

-doc ~"""
```
> otp_doctest:foo().
bar % must fail
```
""".
foo() ->
    foo.

% rebar.config
{profiles, [
    {test, [
        {cover_enabled, true}
    ]}
]}.

% test/otp_doctest_SUITE.erl
-module(otp_doctest_SUITE).
-behaviour(ct_suite).
-compile([export_all, nowarn_export_all]).

all() ->
    [doctests].

doctests(Config) when is_list(Config) ->
    shell_docs:test(otp_doctest, []).

The expected result is to fail, but the test passed:

$ rebar3 ct
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling otp_doctest
===> Running Common Test suites...
%%% otp_doctest_SUITE: .
All 1 tests passed.

After this PR the error is raised:

===> Verifying dependencies...
===> Analyzing applications...
===> Compiling otp_doctest
===> Running Common Test suites...
%%% otp_doctest_SUITE:
%%% otp_doctest_SUITE ==> doctests: FAILED
%%% otp_doctest_SUITE ==> {{badmatch,foo},
 [{erl_eval,expr,6,[{file,"erl_eval.erl"},{line,670}]},
  {shell_docs_test,run_tests,2,[{file,"shell_docs_test.erl"},{line,198}]},
  {lists,foldl,3,[{file,"lists.erl"},{line,2150}]},
  {shell_docs_test,test,2,[{file,"shell_docs_test.erl"},{line,153}]},
  {shell_docs_test,parse_and_run,3,[{file,"shell_docs_test.erl"},{line,137}]},
  {shell_docs_test,'-parse_and_run/3-lc$^0/1-0-',3,
                   [{file,"shell_docs_test.erl"},{line,133}]},
  {shell_docs_test,'-module/2-lc$^0/1-0-',3,
                   [{file,"shell_docs_test.erl"},{line,122}]},
  {shell_docs_test,module,2,[{file,"shell_docs_test.erl"},{line,123}]}]}

cc @garazdawi

[github-actions]

CT Test Results

    2 files     70 suites   1h 7m 53s ⏱️
1 567 tests 1 324 ✅ 243 💤 0 ❌
1 817 runs  1 524 ✅ 293 💤 0 ❌

Results for commit e167d95.

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// Erlang/OTP Github Action Bot

[williamthome]

I think this fix should be in the maint branch, but I changed the base branch and a bunch of commits appeared in this PR.