Convert `ClassModule#comment_location` from an Array of `[comment,
location]` pairs to a Hash of `{ location => [comments] }`.

## Motivation

When a class like `RDoc` is documented across multiple files
(`lib/rdoc.rb`, `lib/rdoc/rubygems_hook.rb`, etc.), each file
contributes a comment. With the old Array of `[comment, location]`
pairs, there was no efficient way to look up or replace comments by
file. This matters for server live-reload (#1620): when a user edits
`lib/rdoc.rb`, we need to clear that file's comment and re-parse — but
preserve comments from other files **in their original order**. The
Array structure made this fragile because removing and re-appending an
entry moved it to the end, changing the order comments appeared on the
rendered page.

The Hash structure solves this: `{ location => [comments] }` allows O(1)
lookup by file, and Ruby hashes preserve insertion order — replacing a
key's value keeps it in position.

A secondary benefit: a class reopened in the same file now preserves all
its comments:

```ruby
# comment1
class A; end
# comment2
class A; end
```

With the old Array, the C parser had a special-case `delete_if` to
deduplicate same-location entries, while Ruby parsers accumulated
duplicates inconsistently. With the new Hash, each location maps to an
array of comments — both comments are preserved and rendered, matching
the cross-file behavior.

## Changes

- `add_comment`: appends to array per location —
`(@comment_location[location] ||= []) << comment`
- `parse`: uses `flat_map` to flatten per-location comment arrays into
Document parts
- `marshal_load` / `merge`: use `group_by(&:file)` to reconstruct arrays
from Documents
- `documented?`, `search_snippet`, `from_module`: updated for new value
shape
- `i18n/text.rb`: handles Hash with array values
- C parser `delete_if` special case removed (hash key naturally
deduplicates by location)