docs: improve visualizations in TTY and PTY explanations

docs/what-is-a-terminal.md

@@ -54,11 +54,17 @@ A shell indirectly communicates with the terminal via the TTY interface to the
 kernel, which manages the actual communication with the terminal.
 
 ```mermaid
-graph TD
-   TTY[TTY device in the kernel, such as /dev/tty0] -- input --> SHELL[Shell Program]
-   TERM[Terminal Device] -- input --> TTY
-   SHELL -- output --> TTY
-   TTY -- output --> TERM
+flowchart LR
+    subgraph Kernel
+        direction LR
+        TTY["TTY device\n(e.g. /dev/tty/0)"] 
+    end
+    
+    subgraph Userspace
+    SHELL["Shell Program (e.g. zsh)"] <-- "input\noutput" --> TTY
+    end
+    TTY  <-- "input\noutput" -->  TE["Terminal Device"]:::td
+    classDef td stroke:#00F,stroke-width:2px
 ```
 
 Again, the TTY interface doesn't provide a way for the shell program to know
@@ -128,13 +134,20 @@ on the associated terminal also needs to respect the setting of `TERM` and use
 an appropriate library to resolve the correct escape sequences.
 
 ```mermaid
-graph TD
-   TTY[TTY device in the kernel, such as /dev/tty0] -- input --> SHELL[Shell Program]
-   SHELL -- output --> TTY
-   APP[Application, such as vim] -- output --> TTY
-   TTY -- input --> APP
-   TTY -- output --> TERM
-   TERM[Terminal Device] -- input --> TTY
+flowchart 
+    subgraph Kernel
+        direction TB
+        TTY["TTY device\n(e.g. /dev/tty/0)"] 
+    end
+    
+    subgraph Userspace
+    SHELL["Shell Program (e.g. zsh)"] <-- input/output --> TTY
+    SHELL -. "starts" .-> APP
+    APP["Application\n(e.g. vim)"] <-- input/output --> TTY
+
+    end
+    TTY  <-- input/output -->  TE["Terminal Device"]:::td
+    classDef td stroke:#00F,stroke-width:2px
 ```
 
 ## What about stdin, stdout and stderr?
@@ -208,11 +221,21 @@ side allowing for passing information about the window size, and the client side
 essentially just being the I/O stream.
 
 ```mermaid
-graph TD
-   PTY[PTY device in the kernel, such as /dev/pts/0] -- input --> SHELL[Shell Program]
-   TE[Terminal Emulator] -- input --> PTY
-   SHELL -- output --> PTY
-   PTY -- output --> TE
+flowchart 
+    subgraph Kernel
+        direction TB
+        PTYC["PTY client\n(e.g. /dev/pts/0)"] 
+        PTYM[PTY master]
+        PTYC <--> PTYM
+    end
+    
+    subgraph Userspace
+    SHELL["Shell Program (e.g. zsh)"] <-- input/output --> PTYC
+    SHELL -. "starts" .-> APP
+    APP["Application\n(e.g. vim)"] <-- input/output --> PTYC
+    PTYM  <-- input/output -->  TE["Terminal Emulator\n(e.g. wezterm)"]:::wezterm
+    classDef wezterm stroke:#00F,stroke-width:2px
+    end
 ```
 
 A *Terminal Emulator* is a program that creates a PTY and then spawns a child