Correctly documented max image and video vars

Added MAX_IMAGE_SIZE and MAX_VIDEO_SIZE and documented their usage.

.env.production.sample

@@ -160,6 +160,12 @@ STREAMING_CLUSTER_NUM=1
 # Maximum number of pinned posts
 # MAX_PINNED_TOOTS=5
 
+# Maximum image and video upload sizes
+# Units are in bytes
+# 1024 bytes equals 1 megabyte
+# MAX_IMAGE_SIZE=8192
+# MAX_VIDEO_SIZE=40960
+
 # LDAP authentication (optional)
 # LDAP_ENABLED=true
 # LDAP_HOST=localhost