Update docs to show how to restore an identity from a seed phrase

[f0i]

Currently the cli-reference docs don't show how to restore an identity from a seed phrase.

It can be done by dfx identity import --seed-file FileContainingSeedPhrase.txt.

This info should probably be added in docs/cli-reference to show up on internetcomputer.org/.../dfx-identity.

Also the description shown by dfx identity --help should state that an identity can also be imported from a seed phrase not only PEM files.
Current output is:

Commands:
[...]
  import         Creates a new identity from a PEM file
[...]