XDG_RUNTIME_DIR is usually in the default socket-dirs (#3285)

fs/share/man/man1/xpra.1

@@ -510,7 +510,7 @@ useful on video content where it saves a considerable amount of bandwidth.
 \fB--socket-dir\fP=\fIDIR\fP
 Location where to write and look for the Xpra socket files.
 The default location varies from platform to platform
-("~/.xpra" on most Posix systems).
+("\fI~/.xpra\fP" and "\fI$XDG_RUNTIME_DIR/xpra\fP" on most Posix systems).
 If unspecified, the first value from \fBsocket-dirs\fP will be used.
 It may also be specified using the XPRA_SOCKET_DIR environment variable.