[ThinLTO][CachePruning] explicitly disable pruning

In https://reviews.llvm.org/rL321077 and https://reviews.llvm.org/D41231 I fixed a regression in the c-api which prevented the pruning from being *effectively* disabled.

However this approach, helpfully recommended by @labath, is cleaner.
It is also nice to remove the weasel words about effectively disabling from the api comments.

Differential Revision: https://reviews.llvm.org/D41497

llvm-svn: 321376
diff --git a/llvm/include/llvm-c/lto.h b/llvm/include/llvm-c/lto.h
index 0fc02f4..55f3e46 100644
--- a/llvm/include/llvm-c/lto.h
+++ b/llvm/include/llvm-c/lto.h
@@ -764,7 +764,7 @@
  * To avoid filling the disk space, a few knobs are provided:
  *  - The pruning interval limits the frequency at which the garbage collector
  *    will try to scan the cache directory to prune expired entries.
- *    Setting to a negative number applies the maximum interval.
+ *    Setting to a negative number disables the pruning.
  *  - The pruning expiration time indicates to the garbage collector how old an
  *    entry needs to be to be removed.
  *  - Finally, the garbage collector can be instructed to prune the cache until
@@ -782,9 +782,9 @@
                                           const char *cache_dir);
 
 /**
- * Sets the cache pruning interval (in seconds). A negative value sets the
- * maximum possible pruning interval. An unspecified default value will be
- * applied, and a value of 0 will be ignored.
+ * Sets the cache pruning interval (in seconds). A negative value disables the
+ * pruning. An unspecified default value will be applied, and a value of 0 will
+ * be ignored.
  *
  * \since LTO_API_VERSION=18
  */