Add a note describing the apparent lack of output file growth during transfer of null bytes with --sparse (#45).

docs/pv.1

@@ -800,6 +800,13 @@ Numbers passed to \*(lq\fB\-\-last\-written\fR\*(rq,
 \*(lq\fB\-\-width\fR\*(rq, \*(lq\fB\-\-height\fR\*(rq,
 \*(lq\fB\-\-average\-rate\-window\fR\*(rq, and \*(lq\fB\-\-remote\fR\*(rq
 must be integers with no suffix.
+.PP
+When \*(lq\fB\-\-sparse\fR\*(rq is in effect and output is being written to
+a file, that file may look like it has stopped growing if inspected with
+\fBls\fR(1) while null bytes are being written, even though \fBpv\fR still
+reports progress.
+This is because of the way sparse output is achieved, and the file will be
+the correct size when the transfer ends.
 .\"
 .SH REPORTING BUGS
 Please report bugs or feature requests via the issue tracker linked from the

docs/pv.1.md

@@ -760,6 +760,12 @@ Numbers passed to "**\--last-written**", "**\--width**",
 "**\--height**", "**\--average-rate-window**", and "**\--remote**" must
 be integers with no suffix.
 
+When "**\--sparse**" is in effect and output is being written to a file,
+that file may look like it has stopped growing if inspected with
+**ls**(1) while null bytes are being written, even though **pv** still
+reports progress. This is because of the way sparse output is achieved,
+and the file will be the correct size when the transfer ends.
+
 # REPORTING BUGS
 
 Please report bugs or feature requests via the issue tracker linked from