This allows plugins to document the commands they add.
The docstring is parsed as a regular expression:
arg:<type>:<name>:<description>\n
Types are defined in commands.arg_types.
Note that this commit removes support for single letter
shortcuts in command options.
If a command is not properly documented, a warning is issued
with print(), because no logger is available at this point.
This is most useful if the user wants to import a significant number of keys,
as every invocation of `importprivkey` results in rewriting the wallet file to disk.
The API of "importprivkey" is left unchanged for the single privkey case.
