Make "water" layers more consistent

Created by: das-g

• The documentation of water_a and water_p explicitly lists man_made=pier and man_made=reservoir_covered when the SQL logic actually copies the value any arbitrary man_made=* (unless natural=* or leisure=*).
• water_p lists man_made=pier twice (for type values pier and lock_gate) although the logic actually just copies the man_made=* value.

For other OSMaxx layers, we only copy verbatim OSM tag values to attribute type after having checked the values with a whitelist, so that the type attribute has a well-defined range.

• The names of the SQL scripts probably shouldn't end in insert_table. (Those for other layers don't.)
• The name of the water_p insertion SQL script should contain water_p, not water_b