Assign the first member when set() meets duplicate keys again

Lookups find the first member of a repeated key again, so set(object, key,
value) assigns that member (keeping the key where it is) and drops the
others, as documented before, and a view taken from object[key] shows the
new value. This undoes the code and documentation changes of 5cbb8e6d6.
Lookups in edited objects (navigation of editable documents) stop at the
first match, like those of parsed objects.

The tests that commit added expect the first member from lookups now. In the
seeded differential test, the documents with repeated keys repeat them after
the real members; the reference holds the first members, which the edits
address and the lookups are compared with, while materialize() is compared
with what parse() makes of the document's dump.

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

docs/mkdocs/docs/api/basic_json_document/erase.md

@@ -86,8 +86,8 @@ view of a *different* document (overloads 1-2 only; overload 3 always starts fro
 
 !!! info "Duplicate keys"
 
-    Overload 1. removes *every* member with `key`, not just the last one that lookups find -- unlike [`set`](set.md),
-    which assigns that member and drops the rest. This is why it returns a count rather than a single view: there may be
+    Overload 1. removes *every* member with `key`, not just the first -- unlike [`set`](set.md), which assigns the
+    first occurrence and drops the rest. This is why it returns a count rather than a single view: there may be
     more than one member removed, or none.
 
 Like [`set`](set.md) and [`push_back`](push_back.md), `erase` never moves an element's *value*: a view still

docs/mkdocs/docs/api/basic_json_document/set.md

@@ -23,8 +23,7 @@ has `set`; calling it on a read-only `basic_json_document` fails to compile (`#!
 
 1. Replaces the value `target` refers to with `value`.
 2. Sets the member `key` of the object `object` to `value`: assigns it if `object` already has a member with this
-   key -- the last one, should the key occur more than once (the member
-   [`operator[]`](../basic_json_view/operator%5B%5D.md) returns), and the other duplicates are then dropped (see the
+   key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the
    [Notes](#notes) below) -- or appends a new member at the end otherwise. A [null](../basic_json_view/is_null.md)
    `object` first becomes an empty object.
 3. Assigns `value` to the element at index `idx` of the array `array`, which must already exist (`#!cpp idx <
@@ -147,13 +146,12 @@ document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_d
 
 !!! info "Duplicate keys"
 
-    If `object` already has more than one member with `key` (2.), `value` is assigned to the *last* one -- the member
-    [`operator[]`](../basic_json_view/operator%5B%5D.md), [`at`](../basic_json_view/at.md), and
-    [`find`](../basic_json_view/find.md) return for reading, so that a view taken from `object["key"]` before the call
-    shows `value` afterward -- and every other member with the same key is removed. The key stays at the
-    position of its *first* occurrence, where [`materialize()`](../basic_json_view/materialize.md) puts it as well. A
-    lookup, an iteration, and `materialize()` of `object` afterward therefore all agree on a single member for `key`. See the
-    [Notes on duplicate keys](../basic_json_view/operator%5B%5D.md#notes) of `operator[]`.
+    If `object` already has more than one member with `key` (2.), the *first* one is assigned `value` and every
+    later member with the same key is removed -- so that a lookup, an iteration, and
+    [`materialize()`](../basic_json_view/materialize.md) of `object` afterward all agree on a single value for
+    `key`, the same way [`operator[]`](../basic_json_view/operator%5B%5D.md) already picks the first occurrence of a
+    duplicate key for reading. See the [Notes on duplicate keys](../basic_json_view/operator%5B%5D.md#notes) of
+    `operator[]`.
 
 Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited
 before switches it from its parsed layout to a growable block holding links to its elements; a later