Fix schema_assets_filter configuration docs

[ittmann]

The callable set as schemaAssetsFilter should return false for tables that you want filtered out

See for example https://github.com/doctrine/dbal/blob/3.6.x/src/Configuration.php#L168 and https://github.com/doctrine/dbal/blob/4.0.x/src/Schema/AbstractSchemaManager.php#L204

[SenseException]

A description about what to expect on true and false would be insightful. Something like "If the function returns true, then..."

[driehle]

Yes, please add something in the writting text as well on how this filter works!

[DIReports]

My intent with this PR was to fix the documentation error.
Documentation should probably be added to the DBAL Configuration documentation and/or Configuration class

With the latest package versions (dbal 3.5.3 and migrations 3.5.5) adding the 'migrations' table to the asset filter breaks doctrine migrations (and not adding it makes the schema-tool when run with the the "--complete" option drop the table)

[DIReports]

Adding a table to the schema_asset_filter will not prevent an entity table from being created, only tables not owned by the DBAL from being dropped, I changed this in the first line.

Also I added some explanation on how the schema_assets_filter works and removed the 'migrations' table from the example as adding it to the asset filter currently breaks doctrine migrations.

[ittmann]

With the latest package versions (dbal 3.5.3 and migrations 3.5.5) adding the 'migrations' table to the asset filter breaks doctrine migrations (and not adding it makes the schema-tool when run with the the "--complete" option drop the table)

Opened issue doctrine/migrations#1317
Adding the migrations table to doctrine (dbal) schema assets filter breaks migrations

[TomHAnderson]

This was removed in 7f112e9