Document USE_PAGER config (#1086)

* Document `USE_PAGER` config

Enhances #783.

* Fix

Author: artur-intech

doc/Configurations.md

@@ -156,6 +156,8 @@ You can see the hash by typing `IRB.conf`. Below are the primary entries:
 - `:USE_TRACER`: Whether to use the
   {IRB tracer}[rdoc-ref:IRB@Tracer];
   initial value: `false`.
+- `:USE_PAGER`: Controls whether pager is enabled.
+  initial value: `true`.
 - `:VERBOSE`: Whether to print {verbose output}[rdoc-ref:IRB@Verbosity];
   initial value: `nil`.
 - `:__MAIN__`: The main IRB object;