replace <outp> command with \outp in docs

Doxygen has been warning that `<outp>` and `</outp>` are not valid XML/HTML commands since commit cb6d5d76 on 20016-04-04. That's primarily because there is at present no way to tell Doxygen to recognize new XML/HTML tags. The actual errors look like this:

```
.../string.doxygen:187: warning: Unsupported xml/html tag </outp> found
```

I hate build errors since they a) cause needless concern, and b) make it harder to notice when I've introduced a new error. So switch from XML/C## style markup to Doxygen style markup for the "outp" annotation.

1 files changed, 2 insertions, 0 deletions

diff --git a/lexicon_filter.in b/lexicon_filter.in
index e9555e8f..13e8b6d7 100644
--- a/lexicon_filter.in
+++ b/lexicon_filter.in
@@ -71,6 +71,8 @@
         /<[^>]*>/ {
             b html
         }
+        # Preprocess specially recognized commands.
+        s/\\outp/@outp/g
         # Process the rest
         b process
     }