Add exec/2: posix_spawn

docs/content/manual/manual.yml

@@ -3271,6 +3271,13 @@ sections:
       builtin outputs its input in raw mode to stder with no additional
       decoration, not even a newline.
 
+      Additionally, jq has support for executing "external filters" provided by
+      other executables. This functionality is provided by `exec` and related
+      filters viz. `system`, but may not be available on all platforms, and does
+      not necessarily integrate well with other jq features. It is intended to
+      perform smaller processing that is otherwise impossible to perform in jq
+      itself.
+
       Most jq builtins are referentially transparent, and yield constant
       and repeatable value streams when applied to constant inputs.
       This is not true of I/O builtins.
@@ -3344,6 +3351,23 @@ sections:
 
           Returns the line number of the input currently being filtered.
 
+      - title: "`exec(path)`, `exec(path; [args…])`"
+        body: |
+
+          Spawns a new process of path with arguments args. Pipes its input
+          converted to a string as the stdin of the process. Outputs an object
+          like `{'out': "", "err": "", "status": 0}`, containing the stdout,
+          stderr, and exit code of the process respectively. If the process
+          exits abnormally due to an unhandled signal, it will have a status of
+          "-1" and a "signal" parameter giving the numeric value of the signal
+          that caused the process to terminate.
+
+      - title: "`system(path)`, `system(path; [args…])`"
+        body: |
+
+          Exactly equivalent to an `exec` call with the same arguments, followed
+          by `| .out | rtrim`.
+
   - title: 'Streaming'
     body: |