Fix documentation grammar and structure in TableMappingInterface

The documentation in TableMappingInterface has grammatical errors and structural issues that make it confusing.

Current problematic text:

The return list is contains the entity field names, not database field
* (i.e. column) names

Problems:

1. Grammar error: "is contains" should be "contains"
2. The return description is mixed into the method description instead of being in the @return tag
3. Missing article "the" before "database field"

Proposed solution:
Move the return description to the @return docblock tag:

@return A list of field names for the given table. Note these are entity field names, not database field (i.e. column) names. To get the mapping of specific entity field to database columns use ::getColumnNames().

This separates the return value documentation from the method description, fixes the grammar, and provides helpful cross-reference to ::getColumnNames().

Credit: Issue identified by chx