📝 fix docs (#5217)

docs/mkdocs/docs/api/basic_json/update.md

@@ -14,6 +14,8 @@ void update(const_iterator first, const_iterator last, bool merge_objects = fals
 When `merge_objects` is `#!c false` (default), existing keys are overwritten. When `merge_objects` is `#!c true`,
 recursively merges objects with common keys.
 
+If the JSON value is `#!json null`, it is implicitly converted to an empty object before the values are inserted.
+
 The function is motivated by Python's [dict.update](https://docs.python.org/3.6/library/stdtypes.html#dict.update)
 function.
 
@@ -28,8 +30,8 @@ iterators (including the `end()` iterator) and all references to the elements ar
 :   JSON object to read values from
 
 `merge_objects` (in)
-:   when `#!c true`, existing keys are not overwritten, but contents of objects are merged recursively (default:
-    `#!c false`)
+:   when `#!c true`, keys that exist in both objects and whose value in the source is itself an object are merged
+    recursively; all other values are overwritten as usual (default: `#!c false`)
 
 `first` (in)
 :   the beginning of the range of elements to insert
@@ -37,6 +39,10 @@ iterators (including the `end()` iterator) and all references to the elements ar
 `last` (in)
 :   the end of the range of elements to insert
 
+## Exception safety
+
+Basic guarantee: if an exception is thrown during the operation, the JSON value may be partially modified.
+
 ## Exceptions
 
 1. The function can throw the following exceptions:
@@ -45,8 +51,6 @@ iterators (including the `end()` iterator) and all references to the elements ar
 2. The function can throw the following exceptions:
     - Throws [`type_error.312`](../../home/exceptions.md#jsonexceptiontype_error312) if called on JSON values other than
       objects; example: `"cannot use update() with string"`
-    - Throws [`invalid_iterator.202`](../../home/exceptions.md#jsonexceptioninvalid_iterator202) if called on an
-      iterator which does not belong to the current JSON value; example: `"iterator does not fit current value"`
     - Throws [`invalid_iterator.210`](../../home/exceptions.md#jsonexceptioninvalid_iterator210) if `first` and `last`
       do not belong to the same JSON value; example: `"iterators do not fit"`
 
@@ -141,6 +145,11 @@ iterators (including the `end()` iterator) and all references to the elements ar
     }
     ```
 
+## See also
+
+- [insert](insert.md) add values to an array/object
+- [merge_patch](merge_patch.md) applies a JSON Merge Patch
+
 ## Version history
 
 - Added in version 3.0.0.