Fix documentation and tests for 'string-distance'

* src/fns.c (Fstring_distance): Doc fix.

* doc/lispref/strings.texi (Text Comparison): Document
'string-distance'.

* etc/NEWS: Fix wording and mark as documented in the manuals.

* test/src/fns-tests.el (test-string-distance): Move from
subr-tests.el and rename.

1 files changed, 2 insertions, 1 deletions

diff --git a/etc/NEWS b/etc/NEWS
index d40f7816b86..32fcdeff918 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -534,7 +534,8 @@ manual for more details.
 +++
 ** New function assoc-delete-all.
 
-** New function string-distance to calculate Levenshtein distance
++++
+** New function 'string-distance' to calculate the Levenshtein distance
 between two strings.
 
 ** 'print-quoted' now defaults to t, so if you want to see