Fix incorrect default "Invite level" (#1371)

changelogs/server_server/newsfragments/1371.clarification

@@ -0,0 +1 @@
+Correct the default invite level definition in the "Checks performed on receipt of a PDU" section.

content/server-server-api.md

@@ -437,21 +437,25 @@ them.
 
 #### Definitions
 
-**Required Power Level** \
-A given event type has an associated *required power level*. This is
-given by the current `m.room.power_levels` event. The event type is
-either listed explicitly in the `events` section or given by either
-`state_default` or `events_default` depending on if the event is a state
-event or not.
-
-**Invite Level, Kick Level, Ban Level, Redact Level** \
-The levels given by the `invite`, `kick`, `ban`, and `redact` properties
-in the current `m.room.power_levels` state. Each defaults to 50 if
-unspecified.
-
-**Target User** \
-For an `m.room.member` state event, the user given by the `state_key` of
-the event.
+Required Power Level
+
+: A given event type has an associated *required power level*. This is given by
+  the current [`m.room.power_levels`](/client-server-api/#mroompower_levels)
+  event. The event type is either listed explicitly in the `events` section or
+  given by either `state_default` or `events_default` depending on if the event
+  is a state event or not.
+
+Invite Level, Kick Level, Ban Level, Redact Level
+
+: The levels given by the `invite`, `kick`, `ban`, and `redact` properties in
+  the current [`m.room.power_levels`](/client-server-api/#mroompower_levels)
+  state. The invite level defaults to 0 if unspecified. The kick level, ban level
+  and redact level each default to 50 if unspecified.
+
+Target User
+
+: For an [`m.room.member`](/client-server-api/#mroommember) state event, the
+  user given by the `state_key` of the event.
 
 {{% boxes/warning %}}
 Some [room versions](/rooms) accept power level values to be represented as