update file system "special names" to correct /usr

Made some other small changes, but this was mainly to "fix" the misleading
"/usr is 'user'" nmemonic.
See https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/usr.html

collections/_file-system/special-filenames.md

@@ -17,26 +17,40 @@ Some filenames have special meaning:
 | name      | meaning                                                              |
 | --------- | -------------------------------------------------------------------- |
 | `.config`<br>`.secrets`   | Filenames beginning with `.` are [hidden files]({{ site.baseurl }}/files/hidden files) and will not normally be displayed when you ask for a directory listing. |
-| `~`       | Your home directory.<br>This is usually the current directory when you first log in. |
+| `~`       | Your home directory.<br>This is the current directory when you first log in. If you make or download any files, this is where you usually save them. |
 | `~olivia` | User `olivia`'s home directory (you won't necessarily have [permission]({{ site.baseurl }}/files/permissions) to look inside it). |
 
 Most Unix systems have conventions for the name of system directories too. A
 few of them are described below. It's a good idea to become familiar with these
 so you can find your way about. Remember these are only conventions, so
-individual systems can — and will — do things differently:
+individual systems might do things a little differently:
 
 | name       | use                                                             |
 | ---------- | --------------------------------------------------------------- |
-| `/bin`     | Binary files — files that contain the instructions the machine follows for implementing commands or programs. |
-| `/home`    | Often the directory containing all the users' home directories. |
-| `/sbin`    | System binaries, like `/bin` but with executables critical for system operation.|
-| `/tmp`     | Temporary files                                                 |
-| `/usr`     | "User" files — generally more for the users than the system.    |
+| `/bin`     | Binary files — files that contain the instructions the machine follows for implementing commands or fundamental utilities. |
+| `/dev`     | Devices — file representations of things like printers and terminals. |
+| `/home`    | The directory containing each user's home directory. For example, Olivia's home directory would be `/home/olivia` (which is the [absolute file path](../absolute-paths) of `~olivia`, above). |
+| `/sbin`    | System binaries, like `/bin` but with executables critical for system operation. |
+| `/tmp`     | Temporary files.                                                |
+| `/usr `<a href="#footnote-1"><sup>1</sup></a>     | Executables, libraries, and shared resources that are not part of the core system. |
 | `/var`     | "Variable" files — files expected to change a lot.              |
 | `/var/log` | System log files.                                               |
 | `/var/tmp` | Temporary files.                                                |
 
-
+Note that these are all conveniently expressed as
+[absolute file paths](../absolute-paths): they are found in the
+[root directory](../root).
+
+<p class="footnote"><sup id="footnote-1">1</sup>
+  Originally, <code class="language-plaintext highlighter-rouge">/usr</code>
+  was an abbreviation for "user" because it contained the users' files, but in
+  today's Unix systems that's what the
+  <code class="language-plaintext highlighter-rouge">/home</code> directory is
+  for. So it's more helpful to remember
+  <code class="language-plaintext highlighter-rouge">/usr</code> as an
+  abbreviation of "user system resources". If you install a new application,
+  it probably ends up here.
+</p>