Fix three stale doc snippets

py_event_loop:get_loop/0 already returns a NIF reference; the docs in
event_loop_architecture.md and process-bound-envs.md were unwrapping
it through a non-existent get_nif_ref/1. Drop the wrapper.

pools.md showed a {default_pool_size, _} sys.config key; the
supervisor reads num_contexts. Use the real key.

Author: benoitc

docs/event_loop_architecture.md

@@ -253,8 +253,7 @@ Define functions and state for async tasks in your process's namespace:
 
 ```erlang
 %% Get event loop reference
-{ok, Loop} = py_event_loop:get_loop(),
-LoopRef = py_event_loop:get_nif_ref(Loop),
+{ok, LoopRef} = py_event_loop:get_loop(),
 
 %% Define async functions in this process's namespace
 ok = py_nif:event_loop_exec(LoopRef, <<"

docs/pools.md

@@ -165,14 +165,15 @@ default = py_context_router:lookup_pool(json, dumps).  %% Function override
 
 ## Configuration
 
-Configure default pool size via application environment:
+Configure the default pool size via the application environment:
 
 ```erlang
 %% sys.config
 [
     {erlang_python, [
-        %% Default pool size (default: erlang:system_info(schedulers))
-        {default_pool_size, 8}
+        %% Number of contexts in the default pool
+        %% (default: erlang:system_info(schedulers))
+        {num_contexts, 8}
     ]}
 ].
 ```

docs/process-bound-envs.md

@@ -125,8 +125,7 @@ The event loop API also supports per-process namespaces. Each Erlang process get
 
 ```erlang
 %% Get the event loop reference
-{ok, Loop} = py_event_loop:get_loop(),
-LoopRef = py_event_loop:get_nif_ref(Loop),
+{ok, LoopRef} = py_event_loop:get_loop(),
 
 %% Define a function in this process's namespace
 ok = py_nif:event_loop_exec(LoopRef, <<"