improve join docs

[donyunardi]

closes #71
move docs from check_joinable to join methods and improve documentation.

[github-actions]

Code Coverage Summary

Filename                 Stmts    Miss  Cover    Missing
---------------------  -------  ------  -------  ---------------------------------------------------------------------------------------------------------------------------------------------
R/chunk.R                  150       3  98.00%   363-369
R/chunks.R                 416      41  90.14%   334, 424, 430-433, 439, 459, 466-467, 484-491, 500, 506-515, 561, 587, 593-594, 596, 612, 621, 630, 672, 675, 699-704, 1070, 1092, 1149, 1202
R/get_eval_details.R       118     118  0.00%    17-188
R/include_css_js.R           7       7  0.00%    12-19
R/qenv-constructor.R        12       0  100.00%
R/qenv-eval_code.R          48       2  95.83%   89, 98
R/qenv-get_code.R           16       0  100.00%
R/qenv-get_var.R            17       0  100.00%
R/qenv-get_warnings.R       14       0  100.00%
R/qenv-join.R               46       0  100.00%
R/qenv-show.R                4       0  100.00%
R/utils.R                   24      19  20.83%   13, 17-38
TOTAL                      872     190  78.21%

Diff against main

Filename    Stmts    Miss    Cover
----------  -------  ------  -------
TOTAL       -        -       -

Results for commit: 6a31ad4

Minimum allowed coverage is 80%

♻️ This comment has been updated with latest results

[github-actions]

Unit Tests Summary

    1 files    10 suites   3s ⏱️
  98 tests   98 ✔️ 0 💤 0 ❌
414 runs  414 ✔️ 0 💤 0 ❌

Results for commit af0bf96.

♻️ This comment has been updated with latest results.

[nikolas-burkoff]

Looks good to me see my comment inline if you want to act on it