Improve builder API doc #11856

Job	Run time
Python ${{ matrix.python }} (Docutils ${{ matrix.docutils }}; free-threaded)	0s
Python ${{ matrix.python }} (Docutils ${{ matrix.docutils }})	0s
Windows	5m 36s
Python 3.10 (Docutils 0.20)	3m 4s
Python 3.10 (Docutils 0.21)	2m 59s
Coverage	0s
Docutils HEAD	3m 5s
Python 3.11 (Docutils 0.20)	3m 1s
Python 3.11 (Docutils 0.21)	2m 26s
LaTeX	4m 41s
Python 3.12 (Docutils 0.20)	3m 5s
Oldest supported	2m 56s
Python 3.12 (Docutils 0.21)	2m 58s
Python 3.13 (Docutils 0.20)	2m 51s
Python 3.13 (Docutils 0.21)	2m 52s
	39m 34s

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