As suggested by jmc@, only include line and column numbers into messages

when they are meaningful, to avoid confusing stuff like this:
$ mandoc /dev/null
mandoc: /dev/null:0:1: FATAL: not a manual
Instead, just say:
mandoc: /dev/null: FATAL: not a manual

Another example this applies to is documents having a prologue,
but lacking a body.  Do not throw a FATAL error for these; instead,
issue a WARNING and show the empty document, in the man(7) case with
the same amount of blank lines as groff does.  Also downgrade mdoc(7)
documents having content before the first .Sh from FATAL to WARNING.

1 files changed, 8 insertions, 4 deletions

diff --git a/mandoc.1 b/mandoc.1
index ddbcb38a..c8b7aa47 100644
--- a/mandoc.1
+++ b/mandoc.1
@@ -498,9 +498,7 @@ parser:
 .Sh DIAGNOSTICS
 Standard error messages reporting parsing errors are prefixed by
 .Pp
-.Sm off
-.D1 Ar file : line : column : \ level :
-.Sm on
+.D1 Nm Ns : Ar file : Ns Ar line : Ns Ar column : level :
 .Pp
 where the fields have the following meanings:
 .Bl -tag -width "column"
@@ -518,6 +516,12 @@ points to the first character of the word.
 The message level, printed in capital letters.
 .El
 .Pp
+The
+.Ar line
+and
+.Ar column
+fields are omitted when meaningless.
+.Pp
 Message levels have the following meanings:
 .Bl -tag -width "warning"
 .It Cm fatal
@@ -557,7 +561,7 @@ The
 utility may also print messages related to invalid command line arguments
 or operating system errors, for example when memory is exhausted or
 input files cannot be read.
-Such messages do not carry the prefix described above.
+Such messages may not carry the prefix described above.
 .Sh COMPATIBILITY
 This section summarises
 .Nm