docs: expand CAST syntax reference for MySQL and MSSQL

[dimitri]

Problem (ref #1538)

The MSSQL CAST section was a dead-end stub:

"Please refer to the MS SQL CAST clause for details."

…pointing to documentation that did not exist. The MySQL CAST section had the right information scattered across bullet lists but lacked a formal synopsis and any worked examples showing what actually happens to the data type.

The confusion surfaces in issue #1538: a user with nvarchar(40) columns found everything collapsing to text and could not figure out from the docs that they needed keep typemod — or what that keyword even did.

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Changes

docs/ref/mssql.rst — CAST section completely replaced

• Synopsis in BNF notation covering both the type and column match forms with their full guard and option syntax
• Root-cause explanation: the built-in defaults map char/nchar/varchar/nvarchar/ntext → text and drop the typemod — the source of the nvarchar(40) → text surprise
• drop typemod / keep typemod documented with explicit before/after: nvarchar(40) → varchar(40) with keep typemod
• Three copy-pasteable examples: preserve all character-type length modifiers; handle nvarchar(max) (precision = −1) separately; override a single column
• Documents every guard available for MSSQL sources (when default, when (typemod_expression), and not null)

docs/ref/mysql.rst — CAST section reorganised

• Replaces the terse two-line synopsis with a full BNF-style Synopsis block
• Adds a Guards subsection covering all six guard forms, including when signed and and not null which were entirely undocumented
• Rewrites each option as a definition-list entry that states what the default behaviour is (e.g. "many built-in rules default to drop typemod") so users know what they get without specifying anything
• Adds an Examples subsection with three representative patterns:
  • Preserving varchar/char length modifiers with keep typemod
  • Mapping tinyint(1) to boolean vs smallint
  • Handling auto_increment with serial/bigserial

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Build

cd docs && make html

Succeeds with no new warnings (two pre-existing warnings unrelated to this change).