ipc: say what the want flag and the capability probe mean now

ipc.h

@@ -3,7 +3,8 @@
 
 /*
  * Key request: [flags, modifiers, key byte 0, ..., key byte 3].
- * Flags request preedit and distinguish lifecycle reset from physical Escape.
+ * The want flag asks for the preedit text: the client then draws it and
+ * the popup does not.  The reset flag is a lifecycle reset, not Escape.
  * The server identifies the connection as the engine owner; that identity is
  * not sent on the wire.
  * Only Mmask modifier bits are retained.
@@ -13,7 +14,9 @@
  * Lengths are little-endian byte counts; fields are at most Ipcfieldmax bytes,
  * so callers read them into char[Ipcfieldmax+1].
  *
- * Capability control is six bytes: [0x80|want, version, 0, 0, 0, 0].
+ * Capability control is six bytes: [0x80|want, version, 0, 0, 0, 0]; the
+ * server always answers it eaten, which tells the client that extension
+ * frames are understood.
  * Caret control is sixteen bytes: [0x80, version, 1, valid, x, y, h],
  * with the signed coordinates and height encoded as little-endian 32-bit words.
  */