docs: Removed and purged uneeded docs. (#2273)

docs/caching/README.md

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+## Object caching
+
+Object caching by turned on by default with following settings
+
+  - Default cache size 8GB, can be changed from environment variable
+    ``MINIO_CACHE_SIZE`` supports both SI and ISO IEC standard forms
+    for input size parameters.
+
+  - Default expiration of entries is 72 hours, can be changed from
+    environment variable ``MINIO_CACHE_EXPIRY`` supportings Go
+    ``time.Duration`` with valid units  "ns", "us" (or "µs"),
+    "ms", "s", "m", "h".
+
+  - Default expiry interval is 1/4th of the expiration hours, so
+    expiration sweep happens across the cache every 1/4th the time
+    duration of the set entry expiration duration.
+
+### Tricks
+
+Setting MINIO_CACHE_SIZE=0 will turn off caching entirely.
+Setting MINIO_CACHE_EXPIRY=0s will turn off cache garbage collections,
+all cached objects will never expire.
+
+### Behavior
+
+Caching happens for both GET and PUT.
+
+- GET caches new objects for entries not found in cache,
+otherwise serves from the cache.
+
+- PUT/POST caches all successfully uploaded objects.
+
+NOTE: Cache is not populated if there are any errors
+      while reading from the disk.
+
+Expiration happens automatically based on the configured
+interval as explained above, frequently accessed objects
+stay alive for significantly longer time due to the fact
+that expiration time is reset for every cache hit.