mod_brotli: clarify that numeric directive values are powers of two

mod_brotli: clarify that numeric directive values are powers of two

BrotliCompressionWindow and BrotliCompressionMaxInputBlock accept
log2 values, but the docs just said "a value between N and M" without
explaining what those numbers mean in practice. Clarify that the
actual sizes are 2^value bytes, with concrete examples (e.g., 18 =
256 KB, 24 = 16 MB). Also improve BrotliCompressionQuality description
with practical guidance. Add examples to all three directives.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1934401 13f79535-47bb-0310-9956-ffa450edef68