Harden nvbench-compare plotting and filter docs

Skip UNKNOWN rows when collecting summary plot entries so non-numeric
fractional differences cannot reach the plotting path. Add a regression test
that exercises compare_benches(..., plot=True) with an UNKNOWN row.

Document the supported pow2 axis-filter syntax and update the CLI help example
to use NAME[pow2]=EXP, matching the parser behavior for axes displayed as 2^N.

* Document when status ???? (UNKNOWN) is emitted
* Clarify --no-color behavior

* nvbench_compare.md: clarify --no-color behavior, fix example

* Document display options in nvbench_compare.md

* Small mention of plotting capabilities in nvbench_compare.md
* Call out that example requires shell with process substitution capabilities

python/scripts/nvbench_compare.py

@@ -2807,7 +2807,11 @@ def compare_benches(
                                 comparison=comparison,
                             )
                         )
-                    if plot:
+                    if (
+                        plot
+                        and comparison.frac_diff is not None
+                        and math.isfinite(comparison.frac_diff)
+                    ):
                         axis_label = format_axis_values(axis_values, axes, axis_filters)
                         if axis_label:
                             label = f"""{cmp_bench["name"]} | {axis_label}"""
@@ -3038,7 +3042,7 @@ def main() -> int:
         dest="filter_actions",
         action=OrderedBenchmarkFilterAction,
         help=(
-            "Filter on axis value, e.g. -a Elements{io}=2^20. Applies to the "
+            "Filter on axis value, e.g. -a Elements{io}[pow2]=20. Applies to the "
             "most recent --benchmark, or all benchmarks if specified before any "
             "--benchmark arguments."
         ),