Fixed reference to wrong field name.
Added documentation of a function arg.
Added a couple references to other parts of the documentation.
Explained how not specifying extraction filename results in automatic
filename generation.
Several other minor clarifications.
- Default extraction limit of 100MB now provided via a tuning script
loaded in local.bro so that command-line Bro is unlimited by default.
- Extraction directory is now created on request of file extraction
rather than unconditionally in bro_init().
