Rename MEILI_MAX_MDB_SIZE env var and --max-mdb-size option flag

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

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+- Title: Rename MAX_MDB_SIZE  configuration variable
+- Start Date: 2021-
+- 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 configuration variable
+
+## 1. Functional Specification
+
+### I. Summary
+
+This specification is written to rename the `MAX_MDB_SIZE` configuration variable.
+
+### 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`
+- Error handling at index creation should be iso with version 0.20.
+
+When I test the current version of meilisearch in 0.21, the http call on `POST /indexes/:index_uid/documents` is successful and an empty index is created whereas 0.20 returns an error on the API side and the index is not created.
+
+```
+{
+    "message": "Impossible to create index; heed error; MDB_MAP_FULL: Environment mapsize limit reached",
+    "errorCode": "index_creation_failed",
+    "errorType": "internal_error",
+    "errorLink": "https://docs.meilisearch.com/errors#index_creation_failed"
+}
+```
+
+### 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.