[m-rev.] for review: Add a hint about trace count file names.

[Peter Wang]

doc/user_guide.texi:
	Under "Coverage testing", say how trace count files are named by
	default.  This repeats information from the section "Generating
	trace counts" but saves the user (me) from wondering if coverage
	testing actually worked -- trace count files are prefixed with
	"." by default so a simple "ls" won't show them.

diff --git a/doc/user_guide.texi b/doc/user_guide.texi
index c7b311d..eccc6d9 100644
--- a/doc/user_guide.texi
+++ b/doc/user_guide.texi
@@ -5636,6 +5636,9 @@ or by including one of the corresponding options
 in the value of the @samp{MERCURY_OPTIONS} environment variable.
 These runs generate a set of trace counts files
 that can be given to the Mercury test coverage tool, the @samp{mcov} program.
+As usual, trace count files are named with the prefix
+ at samp{.mercury_trace_counts} if the @samp{mtc --output-file} option is not
+given.
 @sp 1
 The @samp{mcov} tool is invoked with a command of the form:
 @sp 1