loader: Document that WITH_BEARSSL may need other tweaks

/boot/loader is right up aginst the 500k limit we have to make sure
everything works in a wide variety of environments. However, adding
WITH_BEARSSL can push it over the edge since we are so close to the
limit with it enabled. One may also need to increase LOADERSIZE when
enabling it. It's often safe to go much higher, especially when you
don't plan on using pxeldr. Document this trade off here.

MFC After:		3 days
Sponsored by:		Netflix
Reviewed by:		sjg, markj
Differential Revision:	https://reviews.freebsd.org/D46211

tools/build/options/WITH_BEARSSL

@@ -8,3 +8,22 @@ This library is currently only used to perform
 signature verification and related operations
 for Verified Exec and
 .Xr loader 8 .
+.Pp
+Due to size constraints, one may need to set
+.Va LOADERSIZE
+larger than the
+default 500000, although often loader is under the 500k limit even with
+this option.
+Setting
+.Va LOADERSIZE
+larger than 500000 may cause
+.Xr pxeboot 8
+to be too large to work.
+Careful testing of the loader in the target environment when built with a larger
+limit to establish safe limits is critical because different BIOS environments
+reserve differing amounts of the low 640k space, making a precise limit for
+everybody impossible.
+.Pp
+See also
+.Va WITH_LOADER_PXEBOOT
+for other considerations.

tools/build/options/WITH_LOADER_VERIEXEC

@@ -4,3 +4,5 @@ with support for verification similar to Verified Exec.
 .Pp
 Depends on
 .Va WITH_BEARSSL .
+May require a larger
+.Va LOADERSIZE .