lower or upper case for proper nouns in function names

[videlec]

As mentioned in this sage-devel thread, it would be nice to fix a convention for the typography of proper noun in functions. In other words what should be the standard

1. count_Heegner_points
2. count_heegner_points
3. both (in which case we need to choose which one is an alias of the other)

The developer manual should be updated accordingly.

see also #18975 for case-insensitive search with search_src

Some examples:

• Element: .N() for numerical approximations

• matrix: .LLL() matrix reduction, .LU() and .QR() matrix decomposition, ..., .is_LLL_reduced()

• quadratic forms: Gram_det, Gram_matrix, GHY_mass__maximal, CS_genus_symbol_list, Hessian_matrix, Kitaoka_mass_at_2

• graphs: lex_BFS

• GelfandTsetlinPattern: Tokuyama_coefficient

CC: @dimpase @nathanncohen

Component: misc

Issue created by migration from https://trac.sagemath.org/ticket/18974