chore(docs/admin/infrastructure): call out and link to awsiamrds auth for aws rds (#15955)

Call out AWS IAM RDS db auth and clarify URL encoding requirement in postgres URL

cli/testdata/coder_server_--help.golden

@@ -51,13 +51,15 @@ OPTIONS:
           all available experiments.
 
       --postgres-auth password|awsiamrds, $CODER_PG_AUTH (default: password)
-          Type of auth to use when connecting to postgres.
+          Type of auth to use when connecting to postgres. For AWS RDS, using
+          IAM authentication (awsiamrds) is recommended.
 
       --postgres-url string, $CODER_PG_CONNECTION_URL
           URL of a PostgreSQL database. If empty, PostgreSQL binaries will be
           downloaded from Maven (https://repo1.maven.org/maven2) and store all
           data in the config root. Access the built-in database with "coder
-          server postgres-builtin-url".
+          server postgres-builtin-url". Note that any special characters in the
+          URL must be URL-encoded.
 
       --ssh-keygen-algorithm string, $CODER_SSH_KEYGEN_ALGORITHM (default: ed25519)
           The algorithm to use for generating ssh keys. Accepted values are

cli/testdata/server-config.yaml.golden

@@ -446,7 +446,8 @@ cacheDir: [cache dir]
 # Controls whether data will be stored in an in-memory database.
 # (default: <unset>, type: bool)
 inMemoryDatabase: false
-# Type of auth to use when connecting to postgres.
+# Type of auth to use when connecting to postgres. For AWS RDS, using IAM
+# authentication (awsiamrds) is recommended.
 # (default: password, type: enum[password\|awsiamrds])
 pgAuth: password
 # A URL to an external Terms of Service that must be accepted by users when