Add a note in bn.h that this is not for general-purpose math.

Sadly we do have to keep existing uses working, but let's make it clear
in the documentation that they're not a priority. Also tweak the text
about being limited by memory; we actually impose a tighter limit than
memory alone.

Change-Id: Ibaccd91cd0a1fe354f93f0123497115b649c0630
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/55265
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

include/openssl/bn.h

@@ -136,7 +136,12 @@ extern "C" {
 
 // BN provides support for working with arbitrary sized integers. For example,
 // although the largest integer supported by the compiler might be 64 bits, BN
-// will allow you to work with numbers until you run out of memory.
+// will allow you to work with much larger numbers.
+//
+// This library is developed for use inside BoringSSL, and uses implementation
+// strategies that may not be ideal for other applications. Non-cryptographic
+// uses should use a more general-purpose integer library, especially if
+// performance-sensitive.
 
 
 // BN_ULONG is the native word size when working with big integers.