Clarify documentation on hidden records

This clarifies the documentation regarding the default
value for a records _hidden field.

content/docs/api/db/system-fields/hidden/contents.lr

@@ -7,11 +7,18 @@ type: sysfield
 body:
 
 This field controls if Lektor should process the page into a build artifact.
-By default each page is built into a build artifact (HTML page) and each
-attachment is processed.  This can be prevented by setting `_hidden` to `yes`.
+By default, each page is built into a build artifact (HTML page) and each
+attachment is copied to the output directory.
+This can be prevented for a particular page or attachment by setting its `_hidden` to `yes`.
 
-This also automatically applies to all children of a page unless they
-forcefully override this setting.
+If an explicit value for the `_hidden` field is not set, the value of the `hidden` option in the
+[child configuration] — or, for attachments, [attachment configuration] — section
+of the parent's datamodel is checked.
+If that, too, is not set, then pages, but not attachments, inherit the hidden status of their parent —
+that is, by default, children of hidden pages will not be built, though attachments of hidden pages will.
+
+[child configuration]: https://www.getlektor.com/docs/models/children/#child-configuration (The [children] section of the datamodel)
+[attachment configuration]: https://www.getlektor.com/docs/models/attachments/ (The [attachments] section of the datamodel)
 
 This is useful for more advanced setups like [Single Page Applications
 :ref](../../../../guides/single-page/).

content/docs/models/attachments/contents.lr

@@ -13,9 +13,9 @@ There are a few settings to control these in the `[attachments]` section:
 * `model`: an optional default model that is used for all attachments.
 * `order_by`: controls the ordering of attachments, similar to how this
   works for child pages.
-* `hidden`: if this is set it can override the hidden flag for all
-  attachments at once.  The default as of Lektor 2 is that attachments are
-  not hidden by default.
+* `hidden`: if this is set, attachments will not be built.
+   (Note that any setting here may be overridden on a per attachment basis by
+   setting that attachment's [`_hidden` field](https://www.getlektor.com/docs/models/attachments/).)
 
 !! The `hidden` flag was added in Lektor 2.0 and the default was changed so
 that attachments are not hidden by default, even if the parent is.

content/docs/models/children/contents.lr

@@ -40,7 +40,10 @@ Here are the most important options below `[children]`:
 - `hidden`: if this is set it can override the parent's hidden flag for
   all children.  This is particularly useful if you want to have a folder
   for error pages or other special pages where the parent itself should not
-  be rendered but all children are.  The default is not set.
+  be rendered but all children should.  The default is not set.
+  (Note that an explicit value for a
+  [child's `_hidden` field](/docs/api/db/system-fields/hidden/) will
+  override any value set here.)
 
 !! The `hidden` flag was added in Lektor 2.0