docs: Update documentation for -w to reflect what it actually does.

Fixes #1175.

src/iperf3.1

@@ -316,7 +316,13 @@ test in both directions (normal and reverse), with both the client and
 server sending and receiving data simultaneously
 .TP
 .BR -w ", " --window " \fIn\fR[KMGT]"
-window size / socket buffer size (this gets sent to the server and used on that side too)
+set socket buffer size / window size.
+This value gets sent to the server and used on that side too; on both
+sides this option sets both the sending and receiving socket buffer sizes.
+This option can be used to set (indirectly) the maximum TCP window size.
+Note that on Linux systems, the effective maximum window size is approximately
+double what is specified by this option (this behavior is not a bug in iperf3
+but a "feature" of the Linux kernel, as documented by tcp(7) and socket(7)).
 .TP
 .BR -M ", " --set-mss " \fIn\fR"
 set TCP/SCTP maximum segment size (MTU - 40 bytes)

src/iperf_locale.c

@@ -165,7 +165,9 @@ const char usage_longstr[] = "Usage: iperf3 [-s|-c host] [options]\n"
                            "  -R, --reverse             run in reverse mode (server sends, client receives)\n"
                            "  --bidir                   run in bidirectional mode.\n"
                            "                            Client and server send and receive data.\n"
-                           "  -w, --window    #[KMG]    set window size / socket buffer size\n"
+                           "  -w, --window    #[KMG]    set send/receive socket buffer sizes\n"
+                           "                            (indirectly sets TCP window size)\n"
+
 #if defined(HAVE_TCP_CONGESTION)
                            "  -C, --congestion <algo>   set TCP congestion control algorithm (Linux and FreeBSD only)\n"
 #endif /* HAVE_TCP_CONGESTION */