docs: Add documentation for the TLS key encryption flag

sample-lnd.conf

@@ -61,6 +61,9 @@
 ; use first --tlsextradomain as Common Name instead, if set.
 ; tlsdisableautofill=true
 
+; If set, the TLS private key will be encrypted to the node's seed.
+; tlsencryptkey=true
+
 ; A list of domains for lnd to periodically resolve, and advertise the resolved
 ; IPs for the backing node. This is useful for users that only have a dynamic IP,
 ; or want to expose the node at a domain.