The current docs of the put-mapping Java API is currently broken. It its current
form, it creates an index and uses the whole mapping definition given as a JSON
string as the type name. Since we didn't check the index created in the
IndicesDocumentationIT so far this went unnoticed.

This change adds test to catch this error to the documentation test, changes the
documentation so it works correctly now and adds an input validation to
PutMappingRequest#buildFromSimplifiedDef() which was used internally to reject
calls where no mapping definition is given.

Closes elastic#31906

…mplifiedDef