doc/lua-filters.md: re-add docs for helper functions

These docs are dropped, as the functions are no longer part of
data/pandoc.lua, from which this section is generated. This is only a
temporary fix: a proper fix will have to re-think how this section is
updated.

doc/lua-filters.md

@@ -1344,6 +1344,81 @@ Lua functions for pandoc scripts.
         -- the above is equivalent to
         -- return {{Str = Str}}
 
+[`pipe (command, args, input)`]{#pipe}
+
+:   Runs command with arguments, passing it some input, and
+    returns the output.
+
+    Returns:
+
+    -   Output of command.
+
+    Raises:
+
+    -   A table containing the keys `command`, `error_code`, and
+        `output` is thrown if the command exits with a non-zero
+        error code.
+
+    Usage:
+
+        local output = pandoc.pipe("sed", {"-e","s/a/b/"}, "abc")
+
+[`walk_block (element, filter)`]{#walk_block}
+
+:   Apply a filter inside a block element, walking its
+    contents.
+
+    Parameters:
+
+    `element`:
+    :   the block element
+
+    `filter`:
+    :   a lua filter (table of functions) to be applied
+        within the block element
+
+    Returns: the transformed block element
+
+[`walk_inline (element, filter)`]{#walk_inline}
+
+:   Apply a filter inside an inline element, walking its
+    contents.
+
+    Parameters:
+
+    `element`:
+    :   the inline element
+
+    `filter`:
+    :   a lua filter (table of functions) to be applied
+        within the inline element
+
+    Returns: the transformed inline element
+
+[`read (markup[, format])`]{#read}
+
+:   Parse the given string into a Pandoc document.
+
+    Parameters:
+
+    `markup`:
+    :   the markup to be parsed
+
+    `format`:
+    :   format specification, defaults to \"markdown\".
+
+    Returns: pandoc document
+
+    Usage:
+
+        local org_markup = "/emphasis/"  -- Input to be read
+        local document = pandoc.read(org_markup, "org")
+        -- Get the first block of the document
+        local block = document.blocks[1]
+        -- The inline element in that block is an `Emph`
+        assert(block.content[1].t == "Emph")
+
+
 # Module pandoc.utils
 
 This module exposes internal pandoc functions and utility