Improve documentation of --base option.

doc/ledger3.texi

@@ -1155,6 +1155,8 @@ $ ledger --no-total balance Billable Project
               -50.0m  Project:XYZ
 @end smallexample
 
+@findex C
+
 This example works because ledger already knows how to handle seconds,
 minutes and hours, as part of its time tracking support.  Defining
 other equivalences is simple.  The following is an example that
@@ -6143,7 +6145,8 @@ Strings}).  The default is:
 
 @item --base
 Reduce convertible commodities down the bottom of the conversion, e.g.
-display time in seconds.  This also applies to custom commodity conversions.
+display time in seconds.  This also applies to custom commodity
+conversions (@pxref{Commodity equivalences}).
 
 @item --basis
 @itemx -B