PHPLIB-937: Clarify install steps when not using Composer

Author: jmikola

docs/tutorial/install-php-library.txt

@@ -68,6 +68,9 @@ considerations for Windows are discussed in the
 Installing the Library
 ----------------------
 
+Using Composer
+~~~~~~~~~~~~~~
+
 The preferred method of installing the |php-library| is with
 `Composer <https://getcomposer.org/>`_ by running the following command from
 your project root:
@@ -76,13 +79,6 @@ your project root:
 
    composer require mongodb/mongodb
 
-While not recommended, you may also manually install the library using a source
-archive attached to the
-`GitHub releases <https://github.com/mongodb/mongo-php-library/releases>`_.
-
-Configure Autoloading
-~~~~~~~~~~~~~~~~~~~~~
-
 Once you have installed the library, ensure that your application includes
 Composer's autoloader as in the following example:
 
@@ -96,11 +92,20 @@ Refer to Composer's `autoloading documentation
 <https://getcomposer.org/doc/01-basic-usage.md#autoloading>`_ for more
 information about setting up autoloading.
 
-If you installed the library manually from a source archive, you will need to
-manually configure autoloading:
-
-#. Map the top-level ``MongoDB\`` namespace to the ``src/`` directory
-   using your preferred autoloader implementation.
+Manual Installation Without Composer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-#. Manually require the ``src/functions.php`` file. This is necessary because
-   PHP does not support autoloading for functions.
+While not recommended, you may also manually install the library using a source
+archive attached to the
+`GitHub releases <https://github.com/mongodb/mongo-php-library/releases>`_. When
+installing the library without Composer, you must ensure that all library
+classes *and* functions are loaded for your application:
+
+#. If you are using a `PSR-4 <https://www.php-fig.org/psr/psr-4/>`_ autoloader,
+   map the top-level ``MongoDB\`` namespace to the ``src/`` directory. If you
+   are not using an autoloader, manually require _all_ class files found
+   recursively within the ``src/`` directory.
+
+#. Regardless of whether you are using an autoloader, manually require the
+   ``src/functions.php`` file. This is necessary because PHP does not support
+   autoloading for functions.