update config name and description for low-priority

src/bootstrap/config.toml.example

@@ -152,8 +152,9 @@
 # known-good version of OpenSSL, compile it, and link it to Cargo.
 #openssl-static = false
 
-# Run the build with low priority
-#low_priority = false
+# Run the build with low priority, by setting the process group's "nice" value
+# to +10 on Unix platforms, and by using a "low priority" job object on Windows.
+#low-priority = false
 
 # =============================================================================
 # General install configuration options