Clarify JEMALLOC_OVERRIDE usage

jemalloc-sys/README.md

@@ -153,10 +153,15 @@ hyphens `-` are replaced with underscores `_`(see
 By default this crate builds and links against a vendored version of `jemalloc`. To
 provide your own `jemalloc` version, set `JEMALLOC_OVERRIDE` to point to a built
 version of `jemalloc`. This can point to either a shared or static library. For
-instance: `JEMALLOC_OVERRIDE=/path/to/libjemalloc.a`. The library should be built
-with `jemalloc`'s `--with-jemalloc-prefix=_rjem_`. For static libraries it is also
-recommended to build with `--with-private-namespace=_rjem_`. For more information
-see [`jemalloc/INSTALL.md`][jemalloc_install].
+instance: `JEMALLOC_OVERRIDE=/path/to/libjemalloc.a`.
+
+`JEMALLOC_OVERRIDE` is for advanced usage only. Providing your own library means
+most features of jemalloc-sys will be ignored in build script, and you are
+responsible for compiling the library to match what `jemalloc-sys` expects.
+Especially, handle API prefix as whatever feature
+`unprefixed_malloc_on_supported_platforms` says.
+
+For more information see [`jemalloc/INSTALL.md`][jemalloc_install].
 
 [jemalloc_install]: https://github.com/jemalloc/jemalloc/blob/dev/INSTALL.md#advanced-configuration