Merge pull request grpc#22252 from zackgalbreath/clarify_docs

Documentation improvements

Author: jtattermusch

BUILDING.md

@@ -172,13 +172,23 @@ If you want to build DLLs, run `cmake` with `-DBUILD_SHARED_LIBS=ON`.
 
 ### Dependency management
 
-gRPC's CMake build system provides two modes for handling dependencies.
-* module - build dependencies alongside gRPC.
-* package - use external copies of dependencies that are already available
-on your system.
+gRPC's CMake build system has two options for handling dependencies.
+CMake can build the dependencies for you, or it can search for libraries
+that are already installed on your system and use them to build gRPC.
 
 This behavior is controlled by the `gRPC_<depname>_PROVIDER` CMake variables,
-ie `gRPC_CARES_PROVIDER`.
+e.g. `gRPC_CARES_PROVIDER`. The options that these variables take are as follows:
+
+* module - build dependencies alongside gRPC. The source code is obtained from
+gRPC's git submodules.
+* package - use external copies of dependencies that are already available
+on your system. These could come from your system package manager, or perhaps
+you pre-installed them using CMake with the `CMAKE_INSTALL_PREFIX` option.
+
+For example, if you set `gRPC_CARES_PROVIDER=module`, then CMake will build
+c-ares before building gRPC. On the other hand, if you set
+`gRPC_CARES_PROVIDER=package`, then CMake will search for a copy of c-ares
+that's already installed on your system and use it to build gRPC.
 
 ### Install after build
 

src/cpp/README.md

@@ -70,18 +70,26 @@ also sets up an `add_subdirectory()` rule for you. This causes gRPC to be
 built as part of your project.
 
 ```cmake
+cmake_minimum_required(VERSION 3.15)
+project(my_project)
+
 include(FetchContent)
 FetchContent_Declare(
   gRPC
   GIT_REPOSITORY https://github.com/grpc/grpc
-  GIT_TAG        v1.25.0
+  GIT_TAG        RELEASE_TAG_HERE  # e.g v1.28.0
 )
+set(FETCHCONTENT_QUIET OFF)
 FetchContent_MakeAvailable(gRPC)
 
 add_executable(my_exe my_exe.cc)
 target_link_libraries(my_exe grpc++)
 ```
 
+Note that you need to
+[install the prerequisites](../../BUILDING.md#pre-requisites)
+before building gRPC.
+
 ### git submodule
 If you cannot use FetchContent, another approach is to add the gRPC source tree
 to your project as a