Updates to the documentation for sqlite3_wal_checkpoint_v2() and related
interfaces, including adding many requirements marks.
Futher tweaks to the sqlite3_wal_checkpoint_v2() documentation.
Simplify the sqlite3_wal_checkpoint() documentation.  Add some source code
evidence marks.
Fix over-length comment lines in sqlite.h.in.  No changes to code.
Add evidence marks and assert()s used as evidence for checkpoint requirements.
Add an implementation mark and fix a comment describing the OP_Checkpoint
opcode.