Point users to update(…, true), JSON Pointer defaults, and built-in std::optional (#5804)

A code search of client code found many hand-written versions of
functionality the library already has: recursive object merges
(update(j, true) since 3.10.5), dotted-path getters (value() and
contains() with a JSON Pointer), get_or helpers (value() throws for a
member that is present but null), and adl_serializer specializations
for std::optional (supported since 3.12.0).

- modifying_values.md: describe both modes of update(), add a
  defaults + user settings recipe, and stop recommending Merge Patch
  for recursive merges.
- merge_patch.md: note that null deletes keys, so a merge patch is not
  a general deep merge; link update().
- default_value.md: add "Nested values" (value/contains with a JSON
  Pointer, building a pointer from a dotted path with operator/=) and
  a warning that null and mistyped members are not missing.
- value.md: note that a null member is converted, not replaced by the
  default.
- json_pointer.md: link value/contains/at with JSON Pointers.
- arbitrary_types.md: note that std::optional needs no serializer.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/value.md

@@ -119,6 +119,14 @@ changes to any JSON value.
 
 ## Notes
 
+!!! warning "`null` members are not missing"
+
+    The default value is used only if the key (or JSON Pointer) does not exist. A member that exists but is
+    `#!json null` is converted like any other value, so `#!cpp j.value("k", 0)` throws a
+    [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if `"k"` is `#!json null`. See
+    [Access with default value](../../features/element_access/default_value.md)
+    for alternatives.
+
 !!! warning "Return type"
 
     The value function is a template, and the return type of the function is determined by the type of the provided