[lldb][docs] Update standalone build instructions (#192613)

* LLVM requires CMake 3.20
(https://llvm.org/docs/GettingStarted.html#software) so we do not need
to mention 3.14 anymore.
* CMAKE_BUILD_TYPE was listed twice in one command.
* "ninja" only works when in the build directory or given `-C <dir>`, so
I have changed that to "cmake --build" which works with ninja and other
build tools.

lldb/docs/resources/build.rst

diff --git a/lldb/docs/resources/build.rst b/lldb/docs/resources/build.rst
index 567afdb..85a7b70 100644
--- a/lldb/docs/resources/build.rst
+++ b/lldb/docs/resources/build.rst
@@ -217,18 +217,17 @@
   single one in ``/path/to/llvm-build``
 
 Run CMake with ``-B`` pointing to a new directory for the provided
-build-tree\ :sup:`1` and the positional argument pointing to the ``llvm``
-directory in the source-tree.\ :sup:`2` Note that we leave out LLDB here and only include
-Clang. Then we build the ``ALL`` target with ninja:
+build-tree and the positional argument pointing to the ``llvm``
+directory in the source-tree.\ :sup:`1` Note that we leave out LLDB here and only include
+Clang. Then we build the ``ALL`` target:
 
 ::
 
   $ cmake -B /path/to/llvm-build -G Ninja \
           -DCMAKE_BUILD_TYPE=[<build type>] \
           -DLLVM_ENABLE_PROJECTS=clang \
-          -DCMAKE_BUILD_TYPE=Release \
           [<more cmake options>] /path/to/llvm-project/llvm
-  $ ninja
+  $ cmake --build /path/to/llvm-build
 
 Now run CMake a second time with ``-B`` pointing to a new directory for the
 main build-tree and the positional argument pointing to the ``lldb`` directory
@@ -240,19 +239,16 @@
 ::
 
   $ cmake -B /path/to/lldb-build -G Ninja \
-          -DCMAKE_BUILD_TYPE=Release \
+          -DCMAKE_BUILD_TYPE=[<build type>] \
           -DLLVM_DIR=/path/to/llvm-build/lib/cmake/llvm \
           [<more cmake options>] /path/to/llvm-project/lldb
-  $ ninja lldb lldb-server
+  $ cmake --build /path/to/lldb-build -t lldb lldb-server
 
 If you do not require or cannot build ``lldb-server`` on your platform, simply
 remove it from the Ninja command.
 
 .. note::
 
-   #. The ``-B`` argument was undocumented for a while and is only officially
-      supported since `CMake version 3.14
-      <https://cmake.org/cmake/help/v3.14/release/3.14.html#command-line>`_
    #. If you want to have a standalone LLDB build with tests enabled, you also
       need to pass in ``-DLLVM_ENABLE_RUNTIME='libcxx;libcxxabi;libunwind'`` to your CMake invocation when configuring your LLVM standalone build.