Update migration docs

[Thanhphan1147]

Refine the MAS migration docs

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Manging Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, senior-review-required)
• The changelog is updated with changes that affect the users of the charm.

[github-actions]

Test results for commit 7b2a0c5

Test coverage for 7b2a0c5

Name                                    Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------
src/actions.py                             36      0      6      0   100%
src/auth/__init__.py                        0      0      0      0   100%
src/auth/mas.py                            85      6      2      0    93%   78-80, 97-99
src/backup.py                             175      5     20      2    96%   351-352, 421-422, 479->481, 482
src/backup_observer.py                    151     16     14      0    90%   162-165, 170-173, 210-213, 242-245
src/charm.py                              276     26     62     12    89%   178->180, 183, 257, 310, 314-315, 321-322, 340-341, 365, 372, 447-451, 454-455, 483-485, 505, 535-541, 571-572
src/charm_types.py                         30      0      0      0   100%
src/database_client.py                     57      1      8      4    92%   35, 47->exit, 69->exit, 88->98
src/database_observer.py                   49      4      2      0    92%   61-64
src/exceptions.py                           3      0      0      0   100%
src/matrix_auth_observer.py                59      5      6      0    92%   133-137
src/media_observer.py                      45      4      2      1    89%   60-62, 81
src/observability.py                       13      0      0      0   100%
src/pebble.py                             216     38     50     12    81%   158->exit, 169-173, 218-219, 239-240, 258-261, 322->327, 333-334, 340-341, 343-344, 375, 377, 379, 381, 383, 390-393, 421, 572-588, 630-637
src/redis_observer.py                      39      3      4      0    93%   63-66
src/s3_parameters.py                       22      0      4      0   100%
src/smtp_observer.py                       61      1     14      2    96%   89, 108->113
src/state/__init__.py                       0      0      0      0   100%
src/state/charm_state.py                  144     11     34      7    90%   168, 172, 193, 218, 224, 230, 234-235, 321, 388-389
src/state/mas.py                           79      8      6      3    87%   154, 160-161, 188-190, 205, 225
src/state/validation.py                    36      3      2      0    92%   110-112
src/synapse/__init__.py                     2      0      0      0   100%
src/synapse/workload.py                   151      7     30      2    93%   366-369, 400-401, 449
src/synapse/workload_configuration.py     142     24     34     12    80%   62-63, 127->exit, 131-132, 171-172, 201, 221-222, 255, 264-265, 280, 285-286, 305->310, 311, 329->331, 341-342, 361, 369->371, 371->373, 378-379, 390->397, 400, 420-421
src/user.py                                23      5      2      1    76%   27-29, 56, 73
-----------------------------------------------------------------------------------
TOTAL                                    1894    167    302     58    90%

Static code analysis report

Working... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:00
Run started:2025-05-02 12:57:06.401157

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 9775
  Total lines skipped (#nosec): 6
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):