Add id and element_id to rel reserved props ; update docs

[mariusconjeaud]

590 #623

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% :tada:

Comparison is base (c8fd6f0) 86.25% compared to head (026eac7) 86.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## rc/5.1.2 #750 +/- ## ============================================ + Coverage 86.25% 86.27% +0.02% ============================================ Files 19 19 Lines 2401 2405 +4 ============================================ + Hits 2071 2075 +4 Misses 330 330 ``` | [Files Changed](https://app.codecov.io/gh/neo4j-contrib/neomodel/pull/750?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=neo4j-contrib) | Coverage Δ | | |---|---|---| | [neomodel/\_version.py](https://app.codecov.io/gh/neo4j-contrib/neomodel/pull/750?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=neo4j-contrib#diff-bmVvbW9kZWwvX3ZlcnNpb24ucHk=) | `100.00% <100.00%> (ø)` | | | [neomodel/relationship.py](https://app.codecov.io/gh/neo4j-contrib/neomodel/pull/750?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=neo4j-contrib#diff-bmVvbW9kZWwvcmVsYXRpb25zaGlwLnB5) | `97.43% <100.00%> (+0.13%)` | :arrow_up: |

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