Refine functions.md as suggested during review

See https://github.com/prometheus/prometheus/pull/11404#issuecomment-1631165746

Signed-off-by: Guillaume Berche <guillaume.berche@orange.com>

docs/querying/functions.md

@@ -289,7 +289,7 @@ label_join(up{job="api-server",src1="a",src2="b",src3="c"}, "foo", ",", "src1",
 ## `label_replace()`
 
 For each timeseries in `v`, `label_replace(v instant-vector, dst_label string, replacement string, src_label string, regex string)`
-matches the regular expression `regex` against the value of the label `src_label`. If it
+matches the [regular expression](https://github.com/google/re2/wiki/Syntax) `regex` against the value of the label `src_label`. If it
 matches, the value of the label `dst_label` in the returned timeseries will be the expansion
 of `replacement`, together with the original labels in the input. Capturing groups in the
 regular expression can be referenced with `$1`, `$2`, etc. Named capturing groups in the regular expression can be referenced with `$name` (where `name` is the  capturing group name). If the regular expression doesn't match then the timeseries is returned unchanged.
@@ -300,6 +300,10 @@ This example will return timeseries with the values `a:c` at label `service` and
 label_replace(up{job="api-server",service="a:c"}, "foo", "$1", "service", "(.*):.*")
 ```
 
+This second example has the same effect than the first example, and illustrates use of named capturing groups:
+```
+label_replace(up{job="api-server",service="a:c"}, "foo", "$name", "service", "(?P<name>.*):(?P<version>.*)")
+```
 ## `ln()`
 
 `ln(v instant-vector)` calculates the natural logarithm for all elements in `v`.