docs: fix six @param names that do not match the signatures - #7341

six @param names that no function of that name takes

• sys/refdb_backend.h — the iterator callback documents out, the argument is iter. unlock documents who, which is what rename calls its signature argument; unlock itself takes sig
• utf-conv.h — git_utf8_from_16 documents src_len, an argument only git_utf8_from_16_with_len has. looks like the block was copied from its longer sibling
• path_w32.h — both trim_end and remove_namespace document path while the argument is str
• w32_util.h — filetime_to_timespec documents FILETIME, which is the type, not the name. argument is ft

comments only, nothing else touched

by the way path_w32 also leaves the second argument len undocumented in both, I left that alone to keep the diff to one thing

found with a checker I wrote that reads @param names against the declaration below them, each of these read by hand