Document false negatives from "man -K"

Fixes https://gitlab.com/man-db/man-db/-/issues/14.

* man/man1/man.man1 (Main modes of operation): Document false negatives
due to things like `\-`.
* NEWS.md: Document this.

2 files changed, 4 insertions, 1 deletions

diff --git a/NEWS.md b/NEWS.md
index 5370f8be..2c3f1aa5 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -18,6 +18,8 @@ Improvements:
  * Change section title recommendations in `man(1)` to mention `STANDARDS`
    rather than `CONFORMING TO`, in line with `man-pages(7)`.
  * Add a `STANDARDS` section to `man(1)` itself.
+ * Document that `man -K` may suffer from false negatives as well as false
+   positives.
 
 man-db 2.11.2 (8 January 2023)
 ==============================
diff --git a/man/man1/man.man1 b/man/man1/man.man1
index 463b367a..0e94b3dc 100644
--- a/man/man1/man.man1
+++ b/man/man1/man.man1
@@ -471,7 +471,8 @@ option is used.
 Note that this searches the
 .I sources
 of the manual pages, not the rendered text, and so may include false
-positives due to things like comments in source files.
+positives due to things like comments in source files, or false negatives
+due to things like hyphens being written as "\e-" in source files.
 Searching the rendered text would be much slower.
 .TP
 .if !'po4a'hide' .BR \-l ", " \-\-local\-file