Inconsistent assertion documentation vs command line client

The docs at https://keybase.io/__/command_line/keybase don't match what the command line client offers:

```
$ cat HandDrawnWeb_icon_set.zip | keybase encrypt chris -o HandDrawnWeb_icon_set.zip.asc --assert fingerprint:FBC07D6A97016CB4
info: ...checking identity proofs
error: unknown assertion type: fingerprint

$ cat HandDrawnWeb_icon_set.zip | keybase encrypt chris -o HandDrawnWeb_icon_set.zip.asc --assert keybase:chris
info: ...checking identity proofs
error: unknown assertion type: keybase
Projects ruby-1.9.3-p327$ keybase encrypt --help
usage: keybase encrypt [-h] [-r] [-l] [-a ASSERT] [--batch] [--prompt-remote]
[-s] [-m MESSAGE] [-b] [-o OUTPUT]
them [file]

Positional arguments:
them the username of the receiver
file the file to be encrypted

Optional arguments:
-h, --help Show this help message and exit.
-r, --track-remote remotely track by default
-l, --track-local don't prompt for remote tracking
-a ASSERT, --assert ASSERT
provide a key assertion
--batch batch-mode without interactivity
--prompt-remote prompt for remote tracking
-s, --sign sign in addition to encrypting
-m MESSAGE, --message MESSAGE
provide the message on the command line
-b, --binary output in binary (rather than ASCII/armored)
-o OUTPUT, --output OUTPUT
the output file to write the encryption to
```

The assertions `keybase` and `fingerprint` return an error message. The help message does not mention the actual assertions that are available. After reading the source code, I see `fingerprint` was renamed to `key`. I can't find the `keybase` assertion anymore.

Compare the documentation at https://keybase.io/__/command_line/keybase with the `keybase encrypt --help` output and the assertion handling described in the issue's source-code investigation. Update the documentation so it reflects the assertions actually supported by the command-line client and removes obsolete examples, then verify the help text and documented commands agree.

Written by the indexing model from the issue text.