chore(docs): Document WAL window configuration options (#1050)

docs/api/_ELECTRIC_RESUMABLE_WAL_WINDOW.md

@@ -0,0 +1,7 @@
+Maximum size that the sync service is allowed to use for keeping around old WAL records in Postgres. Measured in bytes unless one of the following suffixes is used: `k` for KB; `K` for KiB; `m` for MB; `M` for MiB; `g` for GB; `G` for GiB.
+
+Normally, Postgres discards WAL records as soon as they are acknowledged by the replica. However, for the sync service to be able to restore its caches after a restart, it needs to hold on to old WAL records since those may contain transactions that affect electrified tables.
+
+Setting this to a low value may lead to clients having to discard their local copy of the server state and restart their replication stream from scratch.
+
+_In the future Electric may store change diffs or employ techniques for compacting the retained WAL records with the goal of extending the time frame during which a client may catch up to the latest server state without discarding its local state._

docs/api/_ELECTRIC_TXN_CACHE_SIZE.md

@@ -0,0 +1,3 @@
+Size of the in-memory cache for transactions the sync service consumes from Postgres over the logical replication stream. Measured in bytes unless one of the following suffixes is used: `k` for KB; `K` for KiB; `m` for MB; `M` for MiB; `g` for GB; `G` for GiB.
+
+This cache is used to quickly catch up newly connected clients to the current state in Postgres. When the sync service restarts, it will repopulate this in-memory cache by streaming already committed transactions from retained Postgres' WAL records, going as far back in time as the size of the WAL window configured with `ELECTRIC_RESUMABLE_WAL_WINDOW` allows.

docs/api/service.md

@@ -53,6 +53,8 @@ import DatabaseUrl from './_DATABASE_URL.md'
 import DatabaseUseIpv6 from './_DATABASE_USE_IPV6.md'
 import ElectricUseIpv6 from './_ELECTRIC_USE_IPV6.md'
 import HttpPort from './_HTTP_PORT.md'
+import ElectricResumableWalWindow from './_ELECTRIC_RESUMABLE_WAL_WINDOW.md'
+import ElectricTxnCacheSize from './_ELECTRIC_TXN_CACHE_SIZE.md'
 import LogLevel from './_LOG_LEVEL.md'
 
 
@@ -103,6 +105,24 @@ Configure how Electric connects to Postgres and exposes its HTTP/WebSocket API.
   <HttpPort />
 </EnvVarConfig>
 
+### ELECTRIC_RESUMABLE_WAL_WINDOW
+
+<EnvVarConfig
+    name="ELECTRIC_RESUMABLE_WAL_WINDOW"
+    defaultValue="2G"
+    example="800M">
+  <ElectricResumableWalWindow />
+</EnvVarConfig>
+
+### ELECTRIC_TXN_CACHE_SIZE
+
+<EnvVarConfig
+    name="ELECTRIC_TXN_CACHE_SIZE"
+    defaultValue="256M"
+    example="2g">
+  <ElectricTxnCacheSize />
+</EnvVarConfig>
+
 ### LOG_LEVEL
 
 <EnvVarConfig