README.md: Reword prebuilt toolchain use, note rust.prebuiltToolchain

Author: MaulingMonkey

README.md

@@ -268,9 +268,13 @@ cargo {
 
 ## Specifying NDK toolchains
 
-When `prebuiltToolchains = false` or for NDK versions before 19, the plugin looks for (and will
-generate) per-target architecture standalone NDK toolchains as generated by
-`make_standalone_toolchain.py`.
+The plugin can either use prebuilt NDK toolchain binaries, or search for (and if missing, build)
+NDK toolchains as generated by `make_standalone_toolchain.py`.
+
+A prebuilt NDK toolchain will be used if:
+1. `rust.prebuiltToolchain=true` in the per-(multi-)project `${rootDir}/local.properties`
+1. `prebuiltToolchain=true` in the `cargo { ... }` block (if not overridden by `local.properties`)
+1. The discovered NDK is version 19 or higher (if not overridden per above)
 
 The toolchains are rooted in a single Android NDK toolchain directory.  In order of preference, the
 toolchain root directory is determined by: