Rename MAX_MDB_SIZE env var and --max-mdb-size option flag (#48)

* Intialize specification

* Apply suggestions from code review

Co-authored-by: Clémentine Urquizar <clementine@meilisearch.com>

* update specification

* add 0.20 error description in specification file

* remove 0.21 current state missing http error

* update title and add date

Co-authored-by: Clémentine Urquizar <clementine@meilisearch.com>

text/0048-rename-max-mdb-size-var.md

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+- Title: Rename MAX_MDB_SIZE env var and --max-mdb-size option flag
+- Start Date: 2021-04-14
+- Specification PR: [#48](https://github.com/meilisearch/specifications/pull/48)
+- MeiliSearch Tracking-issues: [transplant/#206](https://github.com/meilisearch/transplant/issues/206)
+
+
+# Rename MAX_MDB_SIZE env var and --max-mdb-size option flag
+
+## 1. Functional Specification
+
+### I. Summary
+
+This specification is written to rename the `MAX_MDB_SIZE` environnement variable and `--max-mdb-size` option flag to `MAX_INDEX_SIZE` and `--max-index-size`.
+
+### II. Motivation
+
+Since version 0.21 no longer has a main LMDB database for all indexes but one LMBD database per index, the concept of a `main` database no longer exists internally.
+
+
+### III. Additional Materials
+N/A
+
+### IV. Explanation
+
+- Rename `--max-mdb-size` to `--max-index-size`
+- Rename `MAX_MDB_SIZE` to `MAX_INDEX_SIZE`
+
+### V. Impact on Documentation
+
+- Update the [Max MDB Size documentation part](https://docs.meilisearch.com/reference/features/configuration.html#max-mdb-size).
+
+### VI. Impact on SDKs
+N/A
+
+## 2. Technical Aspects
+N/A
+
+## 3. Future Possibilities
+- Provide a way to set a custom size per index.