docs: add docstrings for BumpFeeTxBuilder #1132

Job	Run time
Build and test macOS arm64 wheels (3.8)	4m 37s
Build and test macOS x86_64 wheels (3.8)	4m 20s
Build and test macOS x86_64 wheels (3.9)	4m 31s
Build and test macOS x86_64 wheels (3.10)	6m 12s
Build and test macOS arm64 wheels (3.9)	5m 43s
Build and test macOS x86_64 wheels (3.11)	5m 36s
Build and test macOS arm64 wheels (3.10)	5m 59s
Build and test macOS x86_64 wheels (3.12)	6m 37s
Build and test macOS arm64 wheels (3.11)	5m 36s
Build and test macOS arm64 wheels (3.12)	6m 16s
Build and test Manylinux 2.28 x86_64 wheels (cp38-cp38)	3m 50s
Build and test Manylinux 2.28 x86_64 wheels (cp39-cp39)	3m 58s
Build and test Manylinux 2.28 x86_64 wheels (cp310-cp310)	3m 40s
Build and test Manylinux 2.28 x86_64 wheels (cp311-cp311)	3m 40s
Build and test Manylinux 2.28 x86_64 wheels (cp312-cp312)	3m 56s
Build and test Windows wheels (3.8)	5m 31s
Build and test Windows wheels (3.9)	5m 22s
Build and test Windows wheels (3.10)	5m 29s
Build and test Windows wheels (3.11)	5m 33s
Build and test Windows wheels (3.12)	5m 30s
	1h 41m 56s

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