Improve purlcli help text

[AyanSinhaMahapatra]

See #450 (comment) and #450 (comment)

[drawliin]

@AyanSinhaMahapatra I looked into this in purldb-toolkit, since purlcli now lives there.. From the current code and local help output, the command summaries already seem short and concise, so this issue may already be resolved. Could you confirm whether there is still any expected change here, or whether this can be closed?