+doc #20699 improve docs for akka-http (#20722)

Made imports in docs snippets more complete and a few minor improvements

akka-docs/rst/scala/http/common/marshalling.rst

@@ -118,7 +118,7 @@ If, however, your marshaller also needs to set things like the response status c
 or any headers then a ``ToEntityMarshaller[T]`` won't work. You'll need to fall down to providing a
 ``ToResponseMarshaller[T]`` or a ``ToRequestMarshaller[T]`` directly.
 
-For writing you own marshallers you won't have to "manually" implement the ``Marshaller`` trait directly.
+For writing your own marshallers you won't have to "manually" implement the ``Marshaller`` trait directly.
 Rather, it should be possible to use one of the convenience construction helpers defined on the ``Marshaller``
 companion:
 

akka-docs/rst/scala/http/common/unmarshalling.rst

@@ -76,7 +76,7 @@ Custom Unmarshallers
 
 Akka HTTP gives you a few convenience tools for constructing unmarshallers for your own types.
 Usually you won't have to "manually" implement the ``Unmarshaller`` trait directly.
-Rather, it should be possible to use one of the convenience construction helpers defined on the ``Marshaller``
+Rather, it should be possible to use one of the convenience construction helpers defined on the ``Unmarshaller``
 companion:
 
 .. includecode2:: /../../akka-http/src/main/scala/akka/http/scaladsl/unmarshalling/Unmarshaller.scala

akka-docs/rst/scala/http/introduction.rst

@@ -25,11 +25,14 @@ Akka HTTP was designed specifically as “not-a-framework”, not because we don
 
 Using Akka HTTP
 ---------------
-Akka HTTP is provided in separate jar files, to use it make sure to include the following dependencies::
+Akka HTTP is provided in a separate jar file, to use it make sure to include the following dependency::
 
-  "com.typesafe.akka" %% "akka-http-core" % "@version@" @crossString@
   "com.typesafe.akka" %% "akka-http-experimental" % "@version@" @crossString@
 
+Mind that ``akka-http`` comes in two modules: ``akka-http-experimental`` and ``akka-http-core``. Because ``akka-http-experimental``
+depends on ``akka-http-core`` you don't need to bring the latter explicitly. Still you may need to this in case you rely
+solely on low-level API.
+
 
 Routing DSL for HTTP servers
 ----------------------------

akka-docs/rst/scala/http/routing-dsl/index.rst

@@ -58,7 +58,7 @@ Bind failures
 ^^^^^^^^^^^^^
 For example the server might be unable to bind to the given port. For example when the port
 is already taken by another application, or if the port is privileged (i.e. only usable by ``root``).
-In this case the "binding future" will fail immediatly, and we can react to if by listening on the Future's completion:
+In this case the "binding future" will fail immediately, and we can react to if by listening on the Future's completion:
 
 .. includecode2:: ../../code/docs/http/scaladsl/HttpServerExampleSpec.scala
   :snippet: binding-failure-high-level-example