docs: Update generics book for 2025

- Revised "Substitution Maps" chapter:
  - New "Subclassing" section
  - New "SIL Type Lowering" section
- New "Opaque Result Types" chapter
- Various smaller edits

New PDF will be available shortly at https://download.swift.org/docs/assets/generics.pdf.

Author: slavapestov

docs/Generics/README.md

@@ -18,7 +18,7 @@ It's written in TeX, so to typeset the PDF yourself, you need a TeX distribution
 
 ### Using `make`
 
-Running `make` in `docs/Generics/` will run `pdflatex` and `bibtex` in the right order to generate the final document with bibliography, index and cross-references:
+Running `make` in `docs/Generics/` will run `pdflatex` and `bibtex` in the right order to generate the final document with bibliography, index, and cross-references:
 
 ```
 cd docs/Generics/
@@ -61,16 +61,13 @@ This is a work in progress.
 
 The following chapters need some editing:
 
-- Part II:
-  - Substitution Maps
 - Part IV:
   - Completion
 
 The following chapters are not yet written:
 
 - Part III:
-  - Opaque Return Types
   - Existential Types
 - Part IV:
   - The Property Map
-  - Rule Minimization
+  - Minimization