Added documentation for LIBPROCESS_SSL_ECDH_CURVE environment variable.
Added documentation for LIBPROCESS_SSL_ECDH_CURVE environment variable.

This adds the adequate documentation entry for the new
`LIBPROCESS_SSL_ECDH_CURVE` environment variable, which allow the
configuration of ECDHE key exchange while establishing TLS sessions.

Review: https://reviews.apache.org/r/60996/

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