Add documentation for exec/2

docs/content/manual/manual.yml

@@ -3271,6 +3271,12 @@ 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`, 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 +3350,15 @@ sections:
 
           Returns the line number of the input currently being filtered.
 
+      - title: "`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. The output of the
+          process is coalesced and outputted as a string. Note that this output
+          will include any terminating newlines, so you may want to add an
+          rtrim after this filter, or use a wrapper.
+
   - title: 'Streaming'
     body: |