Update available since tags for 3.12.0 release

Author: bknoles

docs/guide/infinite-scroll.md

@@ -1,6 +1,6 @@
 # Infinite scroll
 
-@available_since rails=master core=2.2.0
+@available_since rails=3.12.0 core=2.2.0
 
 Inertia's infinite scroll feature loads additional pages of content as users scroll, replacing traditional pagination controls. This is great for applications like chat interfaces, social feeds, photo grids, and product listings.
 

docs/guide/merging-props.md

@@ -31,7 +31,7 @@ The `InertiaRails.merge` method will append new items to existing arrays at the
 InertiaRails.merge { items }
 ```
 
-@available_since rails=master core=2.2.0
+@available_since rails=3.12.0 core=2.2.0
 
 You may change this behavior to prepend items instead.
 
@@ -77,7 +77,7 @@ When merging arrays, you may use the `match_on` parameter to match existing item
 InertiaRails.merge(match_on: 'id') { post_data }
 ```
 
-@available_since rails=master core=2.2.0
+@available_since rails=3.12.0 core=2.2.0
 
 You may also use append and prepend with a hash to specify the field to match.