Update book/forms.rst

Fix typo (missing ";") in source code sample for dynamic validation groups
[Process] Doc for output flush methods.
[Process] Rewriting to avoid wrong sentences.
[Process] Update docs for modifications.
document both cache class loader the ApcClassLoader and the XcacheClassLoader
documentation for the DebugClassLoader
documentation for the MapClassLoader class
add an introduction and documentation for the ClassLoader class
add versionadded directives for new classes
fixes thanks to @WouterJ
fix markup of links
Added docs for the changes proposed in symfony/symfony#8908
Update filesystem.rst
Update filesystem.rst
Documented Stopwatch Helper
fix link to Class Loader docs
document the getEncodings() method of the Request class
Fixing wrong parameter name in redirecting in config
add note for the HTML5 date format
[forms] Remove tip for skipping validation group section