Document JSON path access in restriction and projection

src/reference/specs/type-system.md

@@ -475,7 +475,82 @@ The `json` database type:
 - Used as dtype by built-in codecs (`<object@>`, `<hash@>`, `<filepath@store>`)
 - Stores arbitrary JSON-serializable data
 - Automatically uses appropriate type for database backend
-- Supports JSON path queries where available
+- Supports JSON path access in restriction and projection — see below
+
+### JSON path access
+
+A field inside a `json` attribute is addressed as `attribute.path`, in both
+restriction and projection. DataJoint translates the path per backend, so the
+same expression runs on MySQL and PostgreSQL:
+
+```python
+@schema
+class Equipment(dj.Manual):
+    definition = """
+    name  : varchar(32)
+    ---
+    specs : json
+    """
+```
+
+**Restriction** — a mapping whose key carries the path:
+
+```python
+Equipment & {"specs.vendor": "Acme"}
+```
+
+**Projection** — a named attribute whose value is the path:
+
+```python
+Equipment.proj(vendor="specs.vendor")
+```
+
+Both emit `json_value()` on MySQL and `jsonb_extract_path_text()` on
+PostgreSQL. Nested fields and array elements use dots and brackets:
+`specs.probe.serial`, `specs.channels[0]`.
+
+#### Extracted values are text
+
+Both backend functions return text, so a numeric or boolean field comes back as
+a string unless a type is requested:
+
+```python
+Equipment.proj(ch="specs.channels")            # '64'  — str
+Equipment.proj(ch="specs.channels:unsigned")   # 64    — int
+```
+
+The annotation is `path:type`. Portable spellings are `unsigned`, `signed` and
+`decimal(M,D)`.
+
+!!! warning "Two defects to know about when filtering"
+
+    **Compare against a string.** A restriction value that is a Python `int` or
+    `bool` is not coerced to match the extracted text. `& {"specs.calibrated":
+    True}` returns **no rows** on MySQL and raises on PostgreSQL; write
+    `& {"specs.calibrated": "true"}` and `& {"specs.channels": "64"}` until
+    [#1564](https://github.com/datajoint/datajoint-python/issues/1564) is fixed.
+
+    **`:int` is not portable.** It works on PostgreSQL and raises on MySQL, whose
+    `RETURNING` clause does not accept it. Use `unsigned` or `signed`, which work
+    on both — [#1563](https://github.com/datajoint/datajoint-python/issues/1563).
+
+#### Ordering comparisons
+
+A path yields text, so `>` and `<` need a typed projection first:
+
+```python
+big = Equipment.proj(ch="specs.channels:unsigned") & "ch > 32"
+```
+
+#### What is not supported
+
+Comparing a whole JSON object — `& {"specs": {"vendor": "Acme", "channels": 64}}` —
+raises a syntax error. Restrict on a path, or normalize the field into a column.
+
+Hidden attributes — platform-managed columns whose names begin with `_`, such as
+`_job_version` — cannot be reached by the mapping form at all: the predicate is
+dropped and every row is returned
+([#1561](https://github.com/datajoint/datajoint-python/issues/1561)).
 
 ## Built-in Codecs