Cygwin: Make gmondump conform to its doc + adjust doc

The doc for gmondump says 1 or more FILENAME are expected, but 0 is
handled. That's an oversight. Make invocation with 0 FILENAMEs print a
one-line help message.

Reword the beginning of profiler's description doc to clarify target's
child processes are run but only optionally profiled.

winsup/doc/utils.xml

@@ -2240,9 +2240,10 @@ specifying an empty password.
 
     <refsect1 id="profiler-desc">
       <title>Description</title>
-    <para>The <command>profiler</command> utility executes a given program, and
-      optionally the children of that program, collecting the location of the
-      CPU instruction pointer (IP) many times per second. This gives a profile
+    <para>The <command>profiler</command> utility executes a given program and
+      any children of that program, collecting the location of the CPU
+      instruction pointer (IP) many times per second. (It is optional to
+      collect this info from child processes.) This info gives a profile
       of the program's execution, showing where the most time is being spent.
       This profiling technique is called "IP sampling".</para>
 

winsup/utils/gmondump.c

@@ -46,6 +46,14 @@ OPTIONS are:\n\
   exit (where == stderr ? 1 : 0 );
 }
 
+void __attribute__ ((__noreturn__))
+usage1 (FILE *where)
+{
+  fprintf (where, "Usage: %s [OPTIONS] FILENAME...\n", pgm);
+
+  exit (where == stderr ? 1 : 0 );
+}
+
 void
 note (const char *fmt, ...)
 {
@@ -248,6 +256,10 @@ main(int argc, char **argv)
         ;
       }
 
+  if (optind >= argc)
+    /* Print one-line help and exit. */
+    usage1 (ofile);
+
   for (int i = optind; i < argc; i++)
     gmondump1 (argv[i]);