Move `Mt', `Ar', and `Li' handling from mdoc_action.c into mdoc_validate.c.

Clarify that `Mt' gets a default `~' (as per groff 1.20) and document it
in mdoc.7.

Made `Lk' be removed in mdoc_macro.c if it has no arguments.  This fixes
segfaults in mdoc_{term,html}.c that nobody's managed to raise yet.

1 files changed, 3 insertions, 0 deletions

diff --git a/mdoc.7 b/mdoc.7
index a8c83703..ba6ac2f9 100644
--- a/mdoc.7
+++ b/mdoc.7
@@ -2124,6 +2124,9 @@ Examples:
 Format a
 .Dq mailto:
 hyperlink.
+If an argument is not provided, the string
+.Dq \(ti
+is used as a default.
 Its syntax is as follows:
 .Pp
 .D1 Pf \. Sx \&Mt Cm address