Clarify xcorr_max() docstring

[Lewis-Khan]

What does this PR do?

Clarifies the ambiguity in the meaning of the Boolean abs_max in
xcorr_max() by editing docstring.

Why was it initiated? Any relevant Issues?

Resolves #2820.

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
• If the PR is making changes to documentation, docs pages can be built automatically.
  Just remove the space in the following string after the + sign: "+DOCS"
• If any network modules should be tested for the PR, add them as a comma separated list
  (e.g. clients.fdsn,clients.arclink) after the colon in the following magic string: "+TESTS:"
  (you can also add "ALL" to just simply run all tests across all modules)
• All tests still pass.
• Any new features or fixed regressions are be covered via new tests.
• Any new or changed features have are fully documented.
• Significant changes have been added to CHANGELOG.txt .
• First time contributors have added your name to CONTRIBUTORS.txt .

[ThomasLecocq]

not sure the new text is clearer. What about something like : ?

    :param bool abs_max: Determines if the largest value of the correlation function
        is returned, independent of it being the maximum (largest positive value: correlation)
        or minimum (largest negative value: anticorrelation), defaults to `True`.
        If `False`, the maximum returned is determined in the positive side (correlation only).

    :return: **shift, value** - Shift and value of maximum of
        cross-correlation.

``

[Lewis-Khan]

@ThomasLecocq @trichter I have updated the docstring with further clarification as per recommendations. Does that sound a bit clearer?

[trichter]

Thanks @Lewis-Khan @ThomasLecocq