Rollup merge of #22377 - thiagooak:book-glossary, r=steveklabnik

@steveklabnik Trying out the Glossary idea.

Added the paragraph about 'complicated words' because I think it would be useful to those contributing to the book. Maybe this should not be here

Author: Manishearth

src/doc/trpl/SUMMARY.md

@@ -37,3 +37,4 @@
     * [Macros](macros.md)
     * [Compiler Plugins](plugins.md)
 * [Conclusion](conclusion.md)
+* [Glossary](glossary.md)

src/doc/trpl/glossary.md

@@ -0,0 +1,16 @@
+% Glossary
+
+Not every Rustacean has a background in systems programming, nor in computer
+science, so we've added explanations of terms that might be unfamiliar.
+
+### Arity
+
+Arity refers to the number of arguments a function or operation takes.
+
+```rust
+let x = (2, 3);
+let y = (4, 6);
+let z = (8, 2, 6);
+```
+
+In the example above `x` and `y` have arity 2. `z` has arity 3.