Update Documentation for STDOUT logging feature

[AlyRagab]

Describe the solution you'd like
Update the below documentation regarding the STDOUT as an audit logging feature along side with the file_path support
https://www.vaultproject.io/docs/audit

Additional context
Outputting logs to STDOUT is enabled as per this PR so we just need to make it clear in the official docs
PR : #3235

[cipherboy]

This is doc'd on the file audit logging page: https://www.vaultproject.io/docs/audit/file#file_path