split: add -b option; clarify the behaviour of -C option (#2083)

pages/osx/split.md

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+# split
+
+> Split a file into pieces.
+
+- Split a file, each split having 10 lines (except the last split):
+
+`split -l {{10}} {{filename}}`
+
+- Split a file into 5 files. File is split such that each split has same size (except the last split):
+
+`split -n {{5}} {{filename}}`
+
+- Split a file with 512 bytes in each split (except the last split; use 512k for kilobytes and 512m for megabytes):
+
+`split -b {{512}} {{filename}}`