Auto merge of #2411 - sbeckeriv:cargo-cache-doc-update-1609, r=alexcrichton

Per #1609 "clarify how the cache works"

Added in a single line that states that the cached packages are only
removed by running the clean command.

Please let me know if this is the wrong spot, incorrect information or just silly.
Thanks
becker

Author: bors

src/doc/environment-variables.md

@@ -15,7 +15,8 @@ Here are a list of the variables Cargo sets, organized by when it sets them:
 
 * `CARGO_HOME` - Cargo maintains a local cache of the registry index and of git
   checkouts of crates.  By default these are stored under `$HOME/.cargo`, but
-  this variable overrides the location of this directory.
+  this variable overrides the location of this directory. Once a crate is cached
+  it is not removed by the clean command.
 * `CARGO_PROFILE` - If this is set to a positive integer *N*, Cargo will record
   timing data as it runs.  When it exits, it will print this data as a profile
   *N* levels deep.