libcasper.3: document cap_init's single-threaded program requirement

Because it internally forks.

[skip ci]
MFC after:	1 week
Sponsored by:	Axcient
Reviewed by	emaste, imp
Differential Revision: https://reviews.freebsd.org/D38020

lib/libcasper/libcasper/libcasper.3

@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 29, 2022
+.Dd January 10, 2023
 .Dt LIBCASPER 3
 .Os
 .Sh NAME
@@ -96,6 +96,7 @@ The
 .Fn cap_init
 function instantiates a capability to allow a program to access
 the casper daemon.
+It must be called from a single-threaded context.
 .Pp
 The
 .Fn cap_wrap