Update documentation.

doc/vscode-texinfo.texi

@@ -7,6 +7,7 @@
 @c %**end of header
 
 @set vscode-docs-url https://code.visualstudio.com/docs
+@set vscode-api-url  https://code.visualstudio.com/api
 
 @copying
 This manual is for vscode-texinfo (version @value{VERSION}), an extension of
@@ -340,6 +341,56 @@ extension. To manually trigger a refresh, click the status bar item.
 @node HTML Preview
 @section HTML Preview
 
+You can generate the HTML preview of a Texinfo document in Visual Studio Code,
+to see how the document looks like when displayed online.
+
+In the active text editor which contains a Texinfo document, click the ``Show
+Preview'' button on the top right of the editor. A webview will be created in a
+split editor (if not already), and the HTML preview will be displayed there. The
+``Show Preview'' command is also available in command palette, and has a default
+@code{Ctrl+K V} key binding (on GNU/Linux).
+
+The HTML used for preview is generated by @code{makeinfo --html --nosplit}, and
+Texinfo source is read from disk, instead of taken from a
+@url{@value{vscode-api-url}/references/vscode-api#TextDocument,
+@code{vscode.TextDocument}}. The editor may prompt you to save to file when
+trying to display preview of a document which does not yet exist on disk, so
+that the preview can be displayed.
+
+An opened preview automatically tracks the behaviour of the corresponding
+document. When the document is saved, the preview will refresh. When the editor
+holding the document is closed, the preview also closes.
+
+Texinfo documents tend to be rather large, and @code{makeinfo} may take some
+time to finish. When you see an ``(Updating)'' in the title of a preview, it
+means that the preview is being updated.
+
+@menu
+* Custom CSS::     Use custom stylesheets for HTML preview.
+* Goto Node::      Jump to a node in preview.
+* Misc Settings::  Miscellaneous configuration options regarding preview.
+@end menu
+
+
+@node Custom CSS
+@subsection Custom CSS
+
+You can use a custom CSS to make the HTML preview look prettier. To configure
+this, edit the configuration option @code{texinfo.preview.customCSS}. The CSS
+file can either be an online or a local (starting with @code{file://}) resource.
+
+A good example is @url{https://www.gnu.org/software/gnulib/manual.css}, which is
+popular among manuals of GNU projects. (Note: May require some tinkering when
+used with darker editor themes)
+
+
+@node Goto Node
+@subsection Goto Node
+
+
+@node Misc Settings
+@subsection Misc Settings
+
 
 @node Diagnosis
 @section Diagnosis