Just writing "LHS &= "RHS" & "(comment)" might be enough?
When equations include multiple alignment points (
&), this creates blocks of alternatingly right- and left-aligned columns.
An example
$
& norm(abs(f+g)^(p-1) dot f)_1 + norm(abs(f+g)^(p-1) dot g)_1 \
&= norm(abs(f+g)^(p-1) dot (abs(f) + abs(g)))_1
&wide "（积分是线性运算）" \
&>= norm(abs(f+g)^(p-1) dot abs(f+g))_1
&wide "（绝对值的三角不等式）" \
&= norm(abs(f+g)^p)_1 \
&= norm(f+g)_p^p
&wide "（定义）"
$
Note that wide in math means #h(2em).