docs: restore build_target method documentation which went missing

In the refman rewrite, these functions vanished. I noticed this when I
went looking at the docs for extract_all_objects(), or should I say I
tried to go looking.

docs/yaml/objects/build_tgt.yaml

@@ -6,6 +6,39 @@ description: |
   both shared and static library or shared module.
 
 methods:
+- name: extract_objects
+  returns: extracted_obj
+  description: |
+    Returns an opaque value representing the object files generated for those
+    source files. This is typically used to take single object files and link
+    them to unit tests or to compile some source files with custom flags. To
+    use the object file(s) in another build target, use the
+    `objects:` keyword argument to a [[build_target]] or include them in the command
+    line of a [[custom_target]].
+  varargs:
+    name: source
+    type: str | file
+    description: |
+      Source filenames for which the built objects should be extracted.
+    min_varargs: 1
+
+- name: extract_all_objects
+  returns: extracted_obj
+  description: |
+    Acts the same as `extract_objects`, but returns all object files generated
+    by this target.
+
+    By default only objects built for this target are returned to maintain
+    backward compatibility with previous versions.  The default value for the
+    `recursive` kwarg will eventually be changed to `true` in a future version.
+  kwargs:
+    recursive:
+      type: bool
+      description: |
+        Also return objects passed to the `objects` argument of this target.
+      since: 0.46.0
+      default: true
+
 - name: full_path
   returns: str
   description: |
@@ -24,6 +57,13 @@ methods:
     It will be removed once the, also deprecated, corresponding `path()`
     function in the [[@external_program]] object is removed.
 
+- name: private_dir_include
+  returns: inc
+  description: |
+    Returns a value that works like [[include_directories]], but points to the
+    private directory of this target. Usually only needed if an another target
+    needs to access some generated internal headers of this target.
+
 - name: name
   returns: str
   since: 0.54.0