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 possibility to use ContainerAwareTrait

In addition to https://github.com/symfony/symfony/pull/8488
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
Improve documentation of ContentAwareTrait
Removed some unnecessary whitespace
Added docs for the changes proposed in symfony/symfony#8908
Update filesystem.rst
Update filesystem.rst
Add type hinting for directly injected service $templating
fix typo in code example
Documented Stopwatch Helper