docs: add document equations for `se_atten_v2` #5492

Job	Run time
Build C library (2.15, libdeepmd_c.tar.gz)	7m 9s
Build C library (2.14, >=2.5.0rc0,<2.15, libdeepmd_c_cu11.tar.gz)	6m 36s
Test building from C library	1m 0s
Pass building c library	2s
	14m 47s

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