perlapi: Combine Strtod, my_strtod

[khwilliamson]

And clarify the pod

[mauke]

The space is immaterial. As far as C is concerned, char**e and char * * e are the same token sequence and are parsed the same way.¹

I think this line in perl.h is responsible for giving Strtod the extra space:

=for apidoc AmTR|NV|Strtod|NN const char * const s|NULLOK char ** e

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¹ I'd never write char**e (without any spaces) though, because the * is syntactically a unary prefix operator. Fully parenthesized, the declaration would be char (*(*(e))), i.e. it's all grouping to the right.