doc, rename API heading to C API

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'next'

* next:
  doc, rename API heading to C API
  doc, provide some examples in README.md
  minor, map fcntl's O_CREAT and O_RDWR manually to tcbdb's BDBOCREAT and BDBOWRITER