DOC: Update merge copy parameter code and doc
- [ ] closes #4599 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

I have checked merge copy related codes, found that following codes will decide if copy:
```
pandas/pandas/core/internals/concat.py:229
        if needs_copy and not indexers:
            mgr = mgr.copy()
```
The needs_copy parameter is defined by user input, CoW status and concat axis.The indexers parameter is defined by following codes:
```
pandas/pandas/core/reshape/concat.py:666
            for obj in self.objs:
                indexers = {}
                for ax, new_labels in enumerate(self.new_axes):
                    # ::-1 to convert BlockManager ax to DataFrame ax
                    if ax == self.bm_axis:
                        # Suppress reindexing on concat axis
                        continue

                    # 1-ax to convert BlockManager axis to DataFrame axis
                    obj_labels = obj.axes[1 - ax]
                    if not new_labels.equals(obj_labels):
                        indexers[ax] = obj_labels.get_indexer(new_labels)

                mgrs_indexers.append((obj._mgr, indexers))
```
I don't know why new_labels can be not equal to obj_labels. They are both from objs, although there are some operations in self.new_axes function. I have tried several examples, but can't get an indexers value which is not None.
 Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.