User manual updates

[aakash10gupta]

This pull request contains the following updates to the user manual -

• Path updates for parameters moved from constants.h to Par_file and vice versa.
• Other minor corrections.

[buildbot-princeton]

The files changed do not require a buildbot test. The maintainers can commit these changes.

[codecov-commenter]

Codecov Report

Base: 45.68% // Head: 45.68% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (019b1c2) compared to base (abddab2). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #1561 +/- ## ======================================= Coverage 45.68% 45.68% ======================================= Files 249 249 Lines 55316 55316 ======================================= + Hits 25270 25272 +2 + Misses 30046 30044 -2 ``` | [Impacted Files](https://codecov.io/gh/SPECFEM/specfem3d/pull/1561?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SPECFEM) | Coverage Δ | | |---|---|---| | [src/specfem3D/convert\_time.f90](https://codecov.io/gh/SPECFEM/specfem3d/pull/1561/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SPECFEM#diff-c3JjL3NwZWNmZW0zRC9jb252ZXJ0X3RpbWUuZjkw) | `66.29% <0.00%> (+2.24%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SPECFEM). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SPECFEM)

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