docs: explain signed zero in the diff() round-trip exceptions (#5944)

Spell out that numbers comparing equal produce no operation even if one is
zero and the other negative zero, in diff.md and features/json_patch.md.

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

docs/mkdocs/docs/api/basic_json/diff.md

@@ -19,9 +19,10 @@ The equality holds up to [`operator==`](operator_eq.md), so it has the following
 - A value that is not equal to itself cannot round-trip. This applies to `NaN` (also nested in an array or object) and
   to [discarded](is_discarded.md) values: `diff` emits a `replace` operation for them. A `NaN` in the patch is
   serialized as `#!json null`, so a patch that was dumped and parsed again would set `#!json null`.
-- Numbers that compare equal produce no operation, even if their types or signs differ: `diff(1, 1.0)`,
+- Numbers that compare equal produce no operation, even if their types differ or if one is zero and the other is
-  `diff(1u, 1)` (unsigned and signed integer) and `diff(0.0, -0.0)` all return `#!json []`. The patched value is then
+  negative zero: `diff(1, 1.0)` (integer and floating-point), `diff(1u, 1)` (unsigned and signed integer) and
-  the `source` and keeps its number type.
+  `diff(0.0, -0.0)` (positive and negative zero) all return `#!json []`. The patched value is then the `source` and
+  keeps its number type and sign.
 
 ## Parameters
 

docs/mkdocs/docs/features/json_patch.md

@@ -78,8 +78,9 @@ flowchart LR
     - `NaN` and [discarded](../api/basic_json/is_discarded.md) values are not equal to themselves, so the invariant
       cannot hold for them. `diff` emits a `replace` operation for such a value, and a `NaN` in a patch is serialized
       as `#!json null`.
-    - Numbers that compare equal produce no operation even if their types differ: `diff(1, 1.0)`, `diff(1u, 1)`, and
+    - Numbers that compare equal produce no operation even if their types differ or if one is zero and the other is
-      `diff(0.0, -0.0)` return `#!json []`, so the patched value keeps the number type of *source*.
+      negative zero: `diff(1, 1.0)`, `diff(1u, 1)`, and `diff(0.0, -0.0)` return `#!json []`, so the patched value
+      keeps the number type and sign of *source*.
     - A [binary value](binary_values.md) in a patch is serialized as an object
       (`#!json {"bytes":[...],"subtype":null}`), so a patch that was dumped and parsed again no longer holds a binary
       value.