Return the first member of duplicate keys from lookups again

Looking up the last member of a duplicate key cannot stop at a match, so
every lookup scanned the whole object (1.6 to 3.4 times slower for small
objects). operator[](key), at, find, value, contains, count, and JSON
pointer resolution return the first member again, as yyjson and simdjson
do; materialize() and get<map>() keep the last value, as parse().

The documentation says so in the feature page and on each lookup page,
and explains how to get the value parse() would give. The integer index
templates and the discarded chaining of operator[] stay.

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

docs/mkdocs/docs/api/basic_json_view/find.md

@@ -6,7 +6,7 @@ iterator find(const char* key) const;
 iterator find(const string_t& key) const;
 ```
 
-Finds a member with key `key` -- the last one, should the key occur more than once (see
+Finds a member with key `key` -- the first one, should the key occur more than once (see
 [Notes on duplicate keys](operator[].md#notes)). If the value is not an object, or no member has this key,
 [`end()`](end.md) is returned.
 
@@ -26,11 +26,18 @@ No-throw guarantee: this function never throws exceptions.
 ## Complexity
 
 Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after another,
-in document order, scanning all of them, since the last match is wanted. Each comparison first checks the key's length
+in document order, stopping at the first match. Each comparison first checks the key's length
 -- already known from the index, without reading the key bytes -- before comparing its content.
 
 ## Notes
 
+!!! warning "Duplicate keys: the first member wins"
+
+    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
+    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
+    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
+    the last value.
+
 Unlike [`BasicJsonType::find`](../basic_json/find.md), which always returns `#!cpp end()` for a non-object type, this
 also does so for a [discarded](is_discarded.md) view -- there is no separate "invalid" iterator to return.