error.3: ATTRIBUTES: Note functions that are/aren't thread-safe

The markings match glibc markings.
markings of functions in glibc are:
- error:         MT-Safe locale
- error_at_line: MT-Unsafe race:error_at_line/error_one_per_line locale

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/error.3

@@ -118,6 +118,37 @@ is assigned the address of a function
 instead of prefixing the message with the program name and colon.
 The function should print a suitable string to
 .IR stderr .
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lbw37
+l l l.
+Interface	Attribute	Value
+T{
+.BR error ()
+T}	Thread safety	MT-Safe locale
+T{
+.BR error_at_line ()
+T}	Thread safety	T{
+MT-Unsafe race:error_at_line/error_one_per_line locale
+T}
+.TE
+
+The internal
+.I error_one_per_line
+variable is accessed (without any form of synchronization, but since
+it's an int used once, it should be safe enough) and, if
+.I error_one_per_line
+is set nonzero, the internal static variables(not exposed to users)
+used to hold the last printed file name and line number are accessed
+and modified without synchronization; the update is not atomic and it
+occurs before disabling cancellation, so it can be interrupted after
+only one of the two variables is modified. After that,
+.BR error_at_line (3)
+is very much like
+.BR error (3).
 .SH CONFORMING TO
 These functions and variables are GNU extensions, and should not be
 used in programs intended to be portable.