docs+lncfg+sample-lnd.conf: add no-graph-cache option

sample-lnd.conf

@@ -1118,6 +1118,9 @@ litecoin.node=ltcd
 ; a batch of modifications to disk. Defaults to 500 milliseconds.
 ; db.batch-commit-interval=500ms
 
+; Don't use the in-memory graph cache for path finding. Much slower but uses
+; less RAM. Can only be used with a bolt database backend.
+; db.no-graph-cache=true
 
 [etcd]
 
@@ -1170,6 +1173,27 @@ litecoin.node=ltcd
 ; disable.
 ; db.postgres.timeout=
 
+[bolt]
+
+; If true, prevents the database from syncing its freelist to disk. 
+; db.bolt.nofreelistsync=1
+
+; Whether the databases used within lnd should automatically be compacted on
+; every startup (and if the database has the configured minimum age). This is
+; disabled by default because it requires additional disk space to be available
+; during the compaction that is freed afterwards. In general compaction leads to
+; smaller database files.
+; db.bolt.auto-compact=true
+
+; How long ago the last compaction of a database file must be for it to be
+; considered for auto compaction again. Can be set to 0 to compact on every
+; startup. (default: 168h)
+; db.bolt.auto-compact-min-age=0
+
+; Specify the timeout to be used when opening the database.
+; db.bolt.dbtimeout=60s
+
+
 [cluster]
 
 ; Enables leader election if set.
@@ -1217,26 +1241,6 @@ litecoin.node=ltcd
 ; The TLS certificate to use for establishing the remote signer's identity.
 ; remotesigner.tlscertpath=/path/to/remote/signer/tls.cert
 
-[bolt]
-
-; If true, prevents the database from syncing its freelist to disk. 
-; db.bolt.nofreelistsync=1
-
-; Whether the databases used within lnd should automatically be compacted on
-; every startup (and if the database has the configured minimum age). This is
-; disabled by default because it requires additional disk space to be available
-; during the compaction that is freed afterwards. In general compaction leads to
-; smaller database files.
-; db.bolt.auto-compact=true
-
-; How long ago the last compaction of a database file must be for it to be
-; considered for auto compaction again. Can be set to 0 to compact on every
-; startup. (default: 168h)
-; db.bolt.auto-compact-min-age=0
-
-; Specify the timeout to be used when opening the database.
-; db.bolt.dbtimeout=60s
-
 
 [gossip]